Saying nothing about the behavior of the young adults, just assessing the question.
1. As has been noted, they're adults, not kids. So you can't pull them out if they want to stay.
2. If it has passed the point of refund, then there's no question - you basically gave a gift to your children.
3. I can't be 100% sure, but it's very likely that any contribution by the parents to the tuition would be considered a 'gift', and you can't force the young adult to give it back.
However, you can cease contributing more money.
That's assuming there wasn't some sort of contractual pre-nup equivalent.