Q: Using family share with adult children

Yes. With Family Sharing, one adult in the family—the family organizer—agrees to pay for any iTunes, iBooks, and App Store purchases that other family members initiate while they're part of the family group.

They will have their iTunes accounts as they are, because they will only Join to the group with their current ID's. They will keep the balance on their individual Apple ID's.

Sharing purchased content with Family Sharing