Q: Child ID using Debit Card

Do you Have Family Sharing enabled?

According to an article from Apple it has something to do with child online privacy protection laws, and that after setting it up with a credit card you can change your payment back to a debit card.  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.