Q: Family sharing and iTunes gift card credits

Hey Ree079,

It should bill your children’s iTunes credit first before billing your credit card. If that is not the case then report the issue to the iTunes Store by following the steps in the second article. 

Family purchases and payments

http://support.apple.com/en-us/HT201079

Report a problem with an item purchased from the iTunes Store, App Store, Mac App Store, or iBooks Store

http://support.apple.com/en-us/HT204084

Take care,

-Norm G.