Q: Setting up Child on Family Sharing

Is the card you are adding a credit card rather than a debit card?  You have to use a credit card if you are adding a child ID to your family group.

Has something to do with child online privacy laws.  From Family Sharing and Apple IDs for kids:

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.

You're welcome.

This worked for me too, thanks.