Family Sharing: Share payments method with children but not with adults.

We want to be two parents managing two children. Each parent want to share their payment methods with both children, but not with each other. What is the best way to sort this?

The issue is that when i added the mother to the Family-group where i am administrator, Apple decided to move all her subscriptions to my payment method. I want to avoid that, while still being able to pay for items on the children device.

Given the amount of parents with shared custody this should be possible?