Family sharing payment options

We are a family of 3 using family sharing. My wife made some in-app purchases which were charged to my credit card. I understand why this happened but we want to change her purchases to her credit card.

I saw in an older thread that she could ‘add money to her Apple account’ which would then be used for her purchases. However, we could only do so using my credit card (!)

Is there a way for my wife to add money to her account, using her credit card, without disabling purchase sharing (I need that for my son).

Thank you in advance!