Apple One Family Members Billing - I get charged for their App purchases

There is likely nothing that needs fixing.

The payment method for Family Sharing is automatic:

1. If anybody in the family buys or subscribes to something, Apple first attempts to charge the item to that individual's Apple Account (Apple ID) balance, if any.

2. I If a family member does not have enough personal Apple Account balance, any excess will be charged to the Family Organizer's primary payment method (usually a card of some kind). The Organizer's personal balance will not be used for purchases made by other family members. If it cannot bill the primary payment method, the Organizer will need to make another payment method the primary method.

- "Some purchases, including gifts, can't be billed to Apple ID balance and will be charged to the family organizer's payment method." "Some subscriptions might not be charged to Apple ID balance."

Ref:

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

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

- Check your Apple ID balance - https://support.apple.com/HT202359

- Change, add, or remove Apple ID payment methods - https://support.apple.com/HT201266

Think it through.

Back in the good only days you only needed one LP for an entire family to listen to it. Then come mp3 players and "personal listening". The whole Family Sharing system is very likely in response to people having for several years bitterly complained that Apple (and in reality Apple was likely doing this at the behest of media companies who ultimately control what Apple is allowed to sell) required a family to purchase multiple copies of things. So Apple had to come up with a way to make families and the media providers content. It's really a quite a clever system to ensure that sharing is limited to a small "family" type group as determined by trusting all members with one person's card. If you don't trust them enough -- or have enough control over them -- to trust them with your card then they likely aren't "family".