Use of gift cards in family share

That is how it is supposed to work. If anybody in the family makes a purchase it is first charged to their personal balance. If they don't have enough to cover the charge then the rest is taken from the credit card or whatever payment method the Family Organizer has to ensure payments for the family. It won't be taken from your balance unless it is something you buy,

How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359

"Some purchases aren’t charged to your Apple ID balance

If you're in a Family Sharing group, purchases are charged to your personal Apple ID balance. If you don't have enough Apple ID balance to pay for the purchase, the remainder is charged to the family organizer's primary payment method."

Family purchases and payments - Apple Support - https://support.apple.com/HT201079

"Which payment method is charged for family members' purchases?

One adult in the family—the family organizer—pays

for most purchases that family members make. But if you're a family

member and you have your own Apple ID balance, your purchases bill to

your personal Apple ID balance. "

Limnos wrote:

...

It won't be taken from your balance unless it is something you buy,

The only way to use your own balance, would be for you to initiate the purchase yourself.

If a family members initiates the purchase it will use their own balance and then your credit card if they don’t have enough balance to cover the cost of the item.

I am the family organizer for our Family Sharing plan. I use gift cards all the time to pay for my own personal purchases and for the shared iCloud storage.

My wife and daughter also redeem iTunes gift cards (now called Apple Gift cards) and my debit card has been charged once in two years because my daughter went over her Apple ID balance by less than $1.00. Otherwise, we all pay for our own purchases using gift cards.