To create a child account you need, due to child online privacy laws, either : Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support
To create an account for a child under 13* in Family Sharing, you can only use the following payment methods:
- Credit cards
- China UnionPay
- A post-paid mobile phone billing account
After you create the child’s Apple ID, you can change your payment method to a different method, such as a debit card, China UnionPay debit card, or a pre-paid mobile phone billing account.
A debit card is not an acceptable payment method