How to create child account without debit Card?

To create a child account, you have to have a form of billing on the Apple ID that is going to be the organizer of the family sharing. It is then advisable you enable restrictions on the Childs device to prevent them from downloading anything without your permission as well as disable in app purchases. https://support.apple.com/en-us/HT201304 , https://www.apple.com/family-sharing/

You don't need a card to create a child account. Read this article on how to create child Apple IDs: https://support.apple.com/en-hk/HT201084