I have a disabled child and I can’t prevent him making purchases and Apple does not allow me to remove my credit card, please help me thanks.

The following may help: Require a password for App Store and iTunes purchases - Apple Support

See the information about requiring a password. If you are using Family Sharing also see the following from that same support article:

If you use Family Sharing, you can use Ask to Buy to control what kids buy. When a child asks to buy an item or download a free item, the family organizer can approve or decline the request.