Children under the age of 13 cannot be removed from the Family Group, so if the issue is because of Ask to Buy for an child's account, then the workaround wouldn't work.
And based on this support article, it sounds like you can only switch your family group once a year. Not sure if leaving then joining the same group is considered switching your group or not, but could be risky:
Leave Family Sharing - Apple Support
The remedy is to put the credit on the Organizer's account, not the child's account.
With a regular Family Sharing purchase, when the person initiates the purchase, it goes through like any purchase, and checks the balance on the Apple ID doing the purchasing first.
But with Ask to Buy, the "purchase" portion of the transaction has not even begun when the Ask to Buy request is sent. It is not until the "approval" is given that the purchase itself is initiated. And I don't know what would be required to send the logic off to another Apple ID to see if there was a credit before completing the purchase where it was initiated.
But, that's why it does it that way when Ask to Buy is involved.
Cheers,
GB