Q: itunes still billing credit card even though there is credit on account.

By 'parent account' do you mean that you are a family organiser in family sharing ? If you do then you can't use a balance on your account, only the card that's on it - in family sharing only the family members can use their balances (if they don't have a balance then the organiser's card will be charged).

To the children's account yes, it should use it - it's any balance on the family organiser's account that can't be used.