Hi Adam,
I apologize, I'm a bit unclear on the exact nature of the issue you are describing. A credit card is only required during the setup of a child Apple ID in Family Sharing, in order to comply with child online privacy protection laws, and only during the initial creation:
Before you begin, make sure that you're using a credit card as your iTunes Store and App Store payment method. To comply with child online privacy protection laws, you will use the CVV or security code from a valid credit card as part of providing your parental consent. If the card on file is a debit card or another payment method, you’ll be asked to provide a credit card before you can continue. After you create the child's Apple ID, you can change your payment method back to a debit card.
Family Sharing and Apple IDs for kids
Regards,
- Brenden