No Granular Controls For Purchase Sharing

My wife and our son are on our family sharing account. We want the ability for her to be able to use her own credit card for her purchase and still be able to have purchase sharing for our son. But it appears that this is an all or nothing option. If she wants to be able to make purchases that go against her own credit card and I want to be able to have our son's purchases be shared and approved by me and her, then I have to completely disable purchase sharing and it then breaks our son's ability to have this functionality. Why is this so cumbersome, obtuse and unnecessarily complex? It's simple, let her have a default payment option that is either her own credit card or even be able to default to the family's purchase sharing that is linked against my own credit card? Doesn't seem that difficult, but to Apple it is.

Really appreciate it Apple if you'd fix this. I've searched and come across hundreds of family users that have this exact problem.