These are all interpersonal problems rather than gaming ones. Here's how I'd handle each of them.
Same Character I'd tolerate it. Not a big fan of this kind of behavior, but it happens. I think it's a roleplaying maturity thing.
One thing I used last game might help you. I like the list of 100 questions about your character, but didn't want to overwhelm my players with homework. Instead of asking them to fill out the list I asked them all 1 question from it before each game. This worked great because it helped grow their characters as the game progressed and because it was one question at a time they actually put thought into the answers.
I bring this up because some of the questions were useful tools to differentiate the player from the character. Asking them what advice the player would give the character, how the player and character would disagree, etc, was useful for the players who were a little too close to their characters. You could ask everyone this like I did or even ask her alone.
Setting inappropriate powers I'd brainstorm with her. "No, you can't play a robot because they don't exist in the Forgotten Realms. How would you feel about a golem?"
I think the important thing here is to work with her rather than restrict her. The idea isn't to keep her from playing a robot. It's to figure out what she is trying to express by playing a robot and then figure out a setting appropriate way to express the same thing.
Since you are the party who knows the most about both the restrictions and features of a campaign setting, you're in a better position to figure out alternatives to her ideas than she is.
I also find it helpful to use book and movie examples as a good way to communicate the feel of the game you're going for. I was trying to run a gritty and dark campaign, but one of my players wrote a 5 page faery tale for her background. With her permission I altered it, but it was clear that she wanted to play a G/PG character in my R rated game. Neither of us were happy.
Since then I've told my players what books or movies inspired the game I'm planning on running. This doesn't have a perfect success rate, because some players won't read books, but it's drastically cut down the number of mismatched characters I received. If only the Game of Thrones TV show came out before I ran my GoT game, the players might not have tried to act like an adventuring party.
Finally, it is possible that a character can't be fulfilled in a certain setting or even a certain campaign. As GM you need to recognize when this is the case. In this case I'd suggest having the player shelve that character until the next game.
Multitasking Intolerable. It's one thing if she's off screen for a while, but if she's playing she should play. If she doesn't have time for the game or if she isn't interested enough to play 100% of the time, she shouldn't be there.
This is one case where I wouldn't go for the one on one conversation. It would seem like you're bullying her. I'd lay down the law in front of everyone so that it's obvious the same rules apply to everyone.
For how to approach the situation, I think it depends on what kind of multitasking the player is doing. I'm willing to be less than polite to someone who is playing video games at my table. For homework, I'd ask nicely. Homework is an obligation, video games aren't. I'd also offer that the player could skip the game session.
Sometimes players feel like game sessions are an obligation too. I had one player with anxiety issues who was getting stressed out about game, but didn't want to miss hanging out with people. The answer for him was to invite him to come hang out while we game, but have no character of his own. I let him play NPCs when he was up for it. If your player is stressed to attend session and finish homework, try telling her she can stay at the table but not play while she's doing her homework and then when she finishes she can come back in. Tell her that it's not that you don't trust her to handle both, but that it's distracting for everyone else.
Also, to clarify when I say multitasking is intolerable, I mean playing RPGs and doing something else entirely. If you're playing game and looking up spells for your next level or writing your backstory, that's fine by me. Your head is still in game mode. Depending on the game, I might even allow miniature painting, but that's pushing it. (The sort of game where I've seen that done in a reasonable way is 3.5 with long combats. When it's 30+ minutes between turns, putting another shade of red on your cloak is entirely reasonable.)
I have experience bringing kids (my own son and his friends) into RPGs. I have experience with Dungeon World. I have experience with Fate and FAE. However - I do not have experience introducing kids to RPGs with Dungeon World or FAE. Just to be explicitly clear.
With that being said, as the probable instigator of this question, I feel that it is incumbent upon me to at least attempt an answer, so here goes:
Both games are products of a new school of game design. They fall into a category I will call here Fiction First. In neither game is the answer to the question What do you do? present on the character sheet. Ever.
In a Fiction First game, the game is played by talking - it is a conversation, where the GM describes situations and then poses the essential question of roleplaying, What do you do? Players answer and the GM rolls the outcome of their answers back into the fiction and the cycle continues, permutes, and repeats.
Dungeon World
Dungeon World is a very focused game, and it is focused upon the very genre you want to play. It is an excellent game in general, and has been an excellent vehicle for me to introduce new players - character creation is fun and collaborative and results in parties that are connected before the start of the adventure. When I warned before of the different principles, here is what I was warning you about:
Dungeon World is derived from the groundbreaking game Apocalypse World by D. Vincent Baker. One part of the brilliance of AW that shines through in DW is the concept of Moves. Moves encapsulate the mechanics of Dungeon World. You've seen them. Player moves read like this:
When you attack an enemy in melee, roll+Str. ✴On a 10+, you deal your damage to the enemy and avoid their attack. At your option, you may choose to do +1d6 damage but expose yourself to the enemy’s attack. ✴On a 7–9, you deal your damage to the enemy and the enemy makes an attack against you.
That's Hack and Slash, a basic player move, from the DW SRD.
The structure of a move is very important - it defines an inflection point in the fiction - a point at which the mechanics kick in - and describes simply but exactly what to do and how to fold the result back into the fiction. A move is invoked by an action in the fiction that triggers it. But a player may never state, "I Hack and Slash the ogre!" - The player has to describe something that triggers the move instead. This is what I mean about the answer not being on the character sheet. This is confusing to some people, especially those used to selecting options from video game menus or boardgame actions.
But Dungeon World is an asymmetrical game. You, the GM, are not playing the same game as the players are. The GM has moves, yes, but they are not the same as player moves. They do not have a trigger/roll/consequence structure. You, as the GM, will probably never roll dice. I usually have my players roll damage for my monsters and other threats. Your moves are things that just happen in the fiction. You say it, it happens. From the gazeteer:
Whenever everyone looks to you to see what happens choose one of these. Each move is something that occurs in the fiction of the game—they aren't code words or special terms. "Use up their resources" literally means to expend the resources of the characters, for example.
- Use a monster, danger, or location move
- Reveal an unwelcome truth
- Show signs of an approaching threat
- Deal damage
- Use up their resources
- Turn their move back on them
- Separate them
- Give an opportunity that fits a class’ abilities
- Show a downside to their class, race, or equipment
- Offer an opportunity, with or without cost
- Put someone in a spot
- Tell them the requirements or consequences and ask
So - players take fictional actions and trigger moves. You use your moves to create fictional actions and / or situations. If a monster has a move like Call for reinforcements, that's exactly what it is - but there will be no check, no percentage chance that reinforcements hear them - what happens is what makes sense in the story. Are all the other goblins (or whatever) dead? Did they already call for reinforcements? Did the players already eliminate or make a deal with those reinforcements? Are they too far from the lair to hear? That's all up to you and what makes sense given the story so far.
You don't really need much in the way of a planned adventure for Dungeon World, I have found. A juicy starting situation and a little local information, plus whatever you think would be super fun to throw at the players is usually plenty. The events tend to spin off on their own as players act and the rules are engaged.
FAE
Fate Core (and FAE, by extension) is a generic game - not generic in the bad sense of "low quality" or "one-size-fits none". Rather, FAE is generic in that it is designed to create adventurous stories through play, while the genre forms, tropes, settings, and specifics of those stories are outside the scope of the system.
The heart of Fate is Characters - though many mistake the (once-novel, now at least 8 years old) concept of Aspects for the game's core. Aspects are great, and they were once novel, and they changed the way I play everything. But you'll understand it better if you think of it as Characters - thus, the Fate Fractal. From the SRD:
In Fate, you can treat anything in the game world like it’s a character. Anything can have aspects, skills, stunts, stress tracks, and consequences if you need it to.
Aspects are just one piece of Characters, and a very flexible piece, but don't forget you have the whole palette of tools to choose from and get stuck using just aspects.
The brilliance of Fate Core (and therefore FAE, I'm going to stop mentioning it now) is the distillation of everything in the game down into just four actions:
- Attack
- Defend
- Overcome
- Create an Advantage
While Dungeon World provides specific moves that guide play out of the fiction, into the mechanics, and back, Fate sets general guidelines that do the same things. So one set of mechanics governs every interaction between the mechanics and the fiction, dispelling the need for the profusion of systems and subsystems that practically defined early RPGs.
So if your player says, "I want to take careful aim with my crossbow so I can be sure to hit the bad guy and not the hostage!" you take that as Creating an Advantage instead of consulting the book to find out if aiming is a free action, whether the player meets the minimum dexterity requirement, what the aim bonus for their particular crossbow is, etc. If they succeed in creating the advantage, you create the aspect You're in my sights! and proceed with play.
If you need something more complicated - the canonical example is On fire! - you can give it skills, like Burn for example, to take actions (like attacking or moving) with, and a stress track to show how far it is from being put out.
Fate isn't completely agnostic about what kind of stories it is designed to tell. What Fate seeks to model is adventure fiction - fun, exciting stories about interesting people doing risky things. And it does a great job of it.
Conclusion
- Neither game is hard to run or play.
- Neither is particularly expensive
- Both games are more similar to each other than to AD&D
- Both are wonderful games and I would not hesitate to use either one in the situation you describe
- If forced to choose, I would say Dungeon World - because in your instance, you have only a short time to play. If you had all summer, you might find that the broader scope of Fate allowed you to explore more genres.
Best Answer
You're the GM--create an opportunity for win-win and help them find it themselves.
This is a great opportunity to work on some basic conflict resolution with your kids. You should mediate a discussion between them in which you challenge them to find a way for them to free the alien and still stay with the pirates. Given their very young age, you will need to prep the playing field first. To do this, you will have to take advantage of your all-powerful role as GM: You control the NPCs creating this conflict.
So in your next session, create a new conflict either among the pirate NPCs or between the pirates and some outside force (perhaps more aliens) in which it becomes much easier to both release the alien and stay friends with the pirates (example: something threatens the ship and if freed the alien can save the pirates). Once this opportunity for win-win is created, begin the conversation between your kids to find that win-win solution themselves. You can drop hints if they have trouble getting there, and the pirates should remain completely obtuse and oblivious to the solution.
This kind of mediated discussion puts the kids on the same side of the argument, not opposing sides. Instead of defending opposing positions (getting what they want at their sister's expense), they're both looking for a common goal (they both get what they want). This reinforces the attitude (necessary to live in a society) that someone doesn't have to lose out for you to gain. It also reinforces that talking things out can lead to something other than a complete impasse and it gives them the satisfaction of reaching the conclusion themselves (they won't see how much you helped).
Yes, kids need to learn you can't always get what you want. Yes, kids need to learn to compromise. But to learn those things, they first need to learn how to get past the digging-in-their-heels stage (I know adults who never have), and helping them reach a win-win that you have made possible by engineering the story is a way to help them mature to that level.
ALSO IMPORTANT: Moral Development
The scenario you present also has some implications for your children's moral development. These girls are not a 17 and 18 year olds playing Pathfinder's Way of the Wicked campaign for some vicarious change-of-pace fun. These are little girls in the early stages of developing their sense of right and wrong. They do not know with the certainty that you do that it is wrong to imprison innocent people against their will. This is a chance to instill some certainty in that conviction. It would be a very bad idea for you to have an outcome in which the alien stays imprisoned. Whatever the outcome, you want your 4 year old to realize in no uncertain terms that letting that alien go was the right thing to do.
Good luck!