Then why should they be able to win money off adults?
Classic "have your cake and eat it too". If you want to play in tournaments with adult prizes then you should expect adult consequences for misbehavior.
They shouldn't be able to. But either way, kids don't have adult brains, and there's nothing anyone can do about that. They are physically different. You can't expect a 12 year old to dunk a basketball, and you can't expect a 12 year old to think like an adult.