Hi unicornnoir,
We understand that you're running into some problems with making purchases through Family Sharing, and we'd like to help. To get started, can you tell us if there are any error messages that appear when you try to make an in-app purchase, and which device you're using to make the purchase? Also, are all of your Family Sharing group members in the same region? The reason we ask is that App Store purchases should automatically use the gift card balance, if there are enough funds available to cover the balance. Any remaining balance would be billed to the Family Sharing organizer's credit card.
If you're finding that Family Sharing is no longer working for you, any group members with an adult account always have the option of leaving the group if they'd like. Here's how to do that:
Leave Family Sharing
Please let us know what you find, and we'll gladly continue to assist.
Have a great day.