If you make money from the work of kids, you must be 100% fair with them. Giving them a worse cut than the parts of the game industry that are already under fire for not giving a big enough cut is not fair. Setting false expectations for them is not fair. And forcing them to earn $1000 before they can pull money out is not fair. Someone else mentioned that Second Life only requires $10 as their minimum.

But even if you're sure you're being 100% fair, you have to be incredibly clear with those kids about what's going on and how it works, and you should probably also make their parents very clear on it, too.

If you can't commit to that kind of fairness towards someone who can't legally sign contracts because the law recognizes that they don't have the life experience to do it fairly on their own, you shouldn't do it, and you definitely shouldn't base your business on it.