Thanks for the response. There isn't an option to do that. Apple chat support has informed me that when Family Sharing is set up my child's Apple Cash cannot be used to make purchases. It is an either or situation, we can either have family sharing and I have to make all of the purchases, or disable family sharing and then my child can use their Apple Cash balance to make purchases.
But if I purchase a gift card and then redeem it for them, they will be able to use the gift card balance to pay for items without needing me to make the purchase via the Family Sharing.