Hi jennb97,
It sounds like you want to know how a balance on your child's Apple ID account will be used if they're part of a family group, is that right? We'd love to help!
When a family member in a Family Sharing group makes a purchase, their personal Apple ID balance is charged. If there isn't enough balance to cover that purchase, the remainder of the charge is billed to the primary payment method of the family organizer's Apple ID account. Check out the "More about charges to Apple ID balance" part of the following link for more details: How App Store and iTunes Store purchases are billed
If purchase sharing is enabled for your family group, the family organizer agrees to pay for any purchases that family members make while they're part of the family group, and purchases are billed directly to the family organizer's account. More details about that here: Family purchases and payments
If you're the family organizer, you can add multiple payment methods to your Apple ID account, but your family members can only use the first payment method in the list. There are additional details about that under the "Manage payment methods that family members use" part of this page: Change, add, or remove Apple ID payment methods
Take care!