[RPG] buy a good, but cheap D&D board?

Young children are interested in different things than you are. The D&D that they play will not be what you're used to. I've read various threads where fathers played D&D with their kids and they share a few things in common:

1. Length of play is fairly short. 30-60 minutes seems to be typical. If you can hold their attention longer, great, but don't count on it. Plan accordingly.

2. Play often focuses on stuff other than the fighting. One father let his son build his character and parts of the village out of LEGO blocks. Consider swapping miniatures and graph paper for LEGO blocks. Make LEGO monsters, dungeons, and so on. Do it with your children so they get invested in it.

3. Sometimes they're more interested in the tactical game than the role-playing. Let them fight if that's what they want.

4. Sometimes they're more interested in the role-playing than the combat. Let them freeform role-play. Remember that D&D encourages a sort of sociopathic, genocidal approach to problem-solving and that your kids know better already. Their approach to solving problems might be to run away, to plead with the monsters to stop, and so on. Let these approaches work!

5. When in doubt, just ask them what parts they like. Do those parts. Ignore the boring parts.

The item description you bring is unclear. Is "only the most evil of humans could wear such a vile item" a part of the item's mechanical aspects? Or just a description of the item?

If the former, D&D's Use Magical Device skill can bypass it, since it explicitly allows a character to activate an item's power regardless of alignment:

Emulate an Alignment

Some magic items have positive or negative effects based on the user’s alignment. Use Magic Device lets you use these items as if you were of an alignment of your choice.

Other systems that don't have alignments as part of their mechanics will probably not have any such feature.

However, if this is only a part of the item's description, then I would say that any character can wear it - it's not a question of activation - but it's a morally reprehensible thing to do. So your Chaotic Good Bard, if he uses it, is perhaps not as Good as he thinks he is. Is he wearing it because he feels the end justifies the means? Wearing it can further Good, by doing evil? This could lead to a great story around character development and fall from grace. But don't mix game mechanics questions (which should be stated explicitly in the item description) with moral choices.

Without knowing the specific details of the item in question, in the context of the system it's in, it's hard to give an answer.