Family sharing purchases

Our family has purchase sharing enabled and multiple family members have App Store gift card balances loaded to their account. We also have download approval on do each time a child requests a new game/purchase a guardian has to approve or decline. Knowing my child had a sufficient balance to cover their request, I approved. When checking her gift card balance after the purchase it was the same. The purchase was deducted instead from the guardians gift card balance. How can I load gift cards to each child’s account and have those balances be used first?