Gift cards in Apple Music Family Plans
Can Apple Music Family Plan members use iTunes gift cards for purchases instead of charging the Administrators credit card?
The amount of an Apple Gift card will be credited to the Administrators Apple ID account when it is redeemed. All future charges will be charged against that credit balance until it is used up, when used up, the Administrators credit card will start to be used, in that order.
Also, you might want to read this Apple Knowledge base article about family Sharing purchases...