[RPG] Is Wave Echo Cave connected to the Underdark

I'm not familiar with the adventure path you're using, so I can't speak to it specifically, but I can give some general advice.

There are, generally, four ways to handle this.

1. As you mentioned, is let them walk in and die. They'll, hopefully, learn from their mistakes and be more prepared in the future. A lot of people like this kind of play and it's perfectly valid, so long as they have a way to gauge the strength of the area they're going into or some warning that they might be in over their heads.

   However, some players don't like having their characters die and some groups would rather not have to deal with the discontinuity. It can also be a bit immersion breaking for games that are more focused on story, rather than just tactical and mechanical prows. You said that you have younger players and they don't like their characters dying, so it might be best to avoid this method.

2. Come out and warn them outside of the game. "Just a heads up, the area you're going into is meant for higher level characters, so you might want to come back later." You should still give them the choice to go in, if they want to, but then the decision will be entirely on their heads and they'll also be able to react appropriately.

   This method has the lowest chance of ending in tears and the best chance of ending in success, simply because it's straight forward and gives the players the information and opportunity they need to make an informed decision. That said, it's also the most meta-gamey and that can be a turn off for some.

3. You can also edit the campaign so they simply can't get in before they're ready. You said there's a banshee in the canon version that can tell them what they need? Ok, well, in your version she doesn't know. Simple as that. If they've never played the adventure before (and haven't read ahead) then they'll never know they missed anything. They won't learn anything, but they're guaranteed not to get to the cave too early.

4. Don't worry about it. If they get to the cave early, make sure there are some hints that they might be getting in over their heads, but let them go. Then if and when they get into a fight they can't win, let them run. You can also arrange the fight to minimize the chance of them dying, without going easy on them, by doing things like having the monsters prioritize still standing characters over fallen (but still living characters) and active combatants over characters that are trying to evacuate the fallen. If there's something about the cave that means they can't exit it once they enter, you could put a comparable fight (maybe a little bit stronger than average) at it's entrance to serve the same purpose.

   This method is the most game friendly, but it also has the highest chance of someone dying (baring the one where you just let them all die).

Personally, I prefer the fourth method, since it's the most RP friendly and knowing when and how to retreat in an RPG is a good skill. I don't think the players should always win, but by the same token defeat shouldn't default to them dying. There's also the possibility that good tactics will make up for the level difference, in which case they'll feel really good about overcoming a more difficult challenge. Even if they retreat, getting stronger and coming back to beat someone who's already beaten makes for a great sense of accomplishment.

That said, if you're really worried about them dying and you think they won't handle it well, #2 and #3 will serve you quite well.

There are a few reasons for this weapon being what it is. But the main one is simply that stats are based on what makes sense for a weapon. Real blowguns deal trivial damage (unless poison-tipped) and are pretty hard to use properly, requiring some level of training to be accurate with them.

For example, the Trident is another choice of weapon that you'd never pick mechanically.

Of course, the main point to the blowgun is that it's traditionally used to poison people covertly. Especially when the goal is to disable targets without murdering them, it might come in handy. A blowgun with a tranq dart cannot accidentally murder the target, while a tranq dart delivered from a crossbow has a good chance of accidentally killing the target.

And while there is no mechanical property that makes the blowgun easier to hide, its light weight and small size would logically make it more easy to do so, likely reducing the DC of attempting to sneak one past people.

And it's cheap and light; you can have a blowgun and a case of 50 darts for 11gp and 2lbs, while even a light crossbow and 20 bolts would be 26gp and 6lbs.

(Of course, unless you're hunting chickens, the blowgun still isn't a very good choice without poison tipped darts, and the poison is likely to be way more expensive than the delivery mechanism, so it'll remain a very niche weapon either way. Such is the fate of some tools. You wouldn't see a medieval army outfitted with these things either.)