iTunes NOT Using Gift Cards on File
From Apple's "How iTunes Store purchases are billed":


  How are Family Sharing purchases billed? When you initiate a new purchase, it will be billed directly to the family organizer's
  account, unless you have gift or store credit. First, your store
  credit will be used to pay the partial or total bill. The remainder
  will bill to the family organizer. iTunes Store credit on your account
  isn't shared with other family members. So if the family organizer has
  a $50 credit on their account and another family member initiates a
  purchase, it won't deduct from the organizer's iTunes Store credit,
  but will bill the organizer's credit or debit card. The family
  organizer will get the receipts for any of their family member's
  purchases.


More important legal verbiage about the iTunes Store at this link.