From How to share apps and purchases with your family - Apple Support:
Which payment method is charged for purchases that family members make?
When purchase sharing is turned on, everyone's purchases bill to the family organizer's payment method.* The family organizer can:
When a family member first makes a purchase on their device, the family organizer might need to verify the payment method by entering the CVV.
* If you're in a Family Sharing group, purchases that you make are charged to your personal Apple Account balance. If you don't have enough Apple Account balance to pay for the purchase, the remainder is charged to the family organizer if purchase sharing is turned on.
You (or they) can add store credit to their account so it is used instead of your payment method, but when purchase sharing is enabled the family organizer's payment is the fall back for everyone else in the group. They don't get to spend your store credit.
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