Why can't I use MY apple cash to pay for in-app purchase requests from my child?

Hello Dr_Jen,

This page can help out with providing some details on which payment method is charged for purchases made by others in the Family Sharing group: How to share apps and purchases with your family - Apple Support

Which payment method is charged for purchases that family members make?

When purchase sharing is turned on, everyone's purchases bill to the family organizer's payment method.* The family organizer can:

• Update their payment method or add a new one if there's an issue.

• Check their purchase history to see purchases made by family members.

• Learn which payment methods are available in your region.

When a family member first makes a purchase on their device, the family organizer might need to verify the payment method by entering the CVV. 

• If you're in a Family Sharing group, purchases that you make are charged to your personal Apple Account balance. If you don't have enough Apple Account balance to pay for the purchase, the remainder is charged to the family organizer if purchase sharing is turned on.

If the purchase isn't covered fully by the account balance it may skip that method and head to the next one as well in the list.

You may also wish to reach out to the Apple Support Advisors that specialize in billing concerns here: Apple Support

Cheers.