You should be able to log into their account and redeem the gift card onto that account, it should then only be available for him/her to spend - if using your computer's iTunes then log out of our account via the Store > Sign Out menu option, that will then change to a Store > Sign In option
For how family sharing purchases are billed : Family purchases and payments - Apple Support
After you set up your family, any time a family member initiates a new purchase it will be billed directly to your account unless that family member has gift or store credit. First, their store credit will be used to pay the partial or total bill. The remainder will bill to the family organizer's card. As the family organizer, any receipts generated by the transaction will be sent to you. Learn more about how iTunes Store purchases are billed.