I am 30 years old, I am more than capable of making my own payment decisions and should have that right to use my own money.
It’s kind of a mess on the payment systems but once you wrap your head around it, it’s pretty simple to deal with.
When you join a family plan the organizer’s card is designated for purchases.
With the exceptions:
subscriptions and purchases by a family member’s appleID will still prefer funding from a personal Apple Store balance. (Load with gift cards or purchase directly via the iCloud menu)
It will also fund preferentially from the family member’s Apple Cash account which can be funded in wallet from your debit/credit card. The Apple Cash balance is a bit more flexible as you can also spend that with Apple Pay vs the App Store balance which is locked in to App Store spending and cannot be spent or transferred to anyone in any other way.
As the family organizer you can make sure it’s locked down and not open spending season by NOTloading ANY debit/credit card in your Apple account. You can keep them in Apple Pay and then fund a balance in Apple Cash - the one you can spend with Apple Pay in brick and mortars or companies other than Apple which Apple will treat as though it is the debit/credit card associated with the family plan if none other is available.
I’ve experienced this from the family organizer’s side of things and it is also frustrating being in that position. I am glad they make it so I must authorize purchases as many times a family member’s offspring has gotten ahold of a device and thought it was time to spend some serious bucks on in game purchases. I actually removed all my cards from the App Store as above and started funding it via the Apple Cash method so anyone making big purchases must do the same or their purchases fail for their appleID beyond what I keep funded there. I only keep it funded enough to cover charges i know are due soon
Apple could fix this by publishing a smooth flowchart with great graphics to educate users the orders of operation involved for funding when involved in a family account. I managed to sort it out after countless apple support contacts.
It would also cost them next to nothing to allow family members to opt in to the license / services sharing portion of the relationship but always fund from personal payment methods. But Apple is - well kind of being Apple on that subject.