How do you remove the child from the Apple Cash

You can lock their account. See instructions in View and limit your child or teen's Apple Cash activity - Apple Support

Here is additional information

To set up Apple Cash Family you must first close any accounts that children already had. Make sure that the balance is zero then go here - Manage your Apple Cash account - Apple Support next, go here - Set up and use Apple Cash Family. (Additional support links View and limit your child or teen's Apple Cash activity, Leave Apple Cash Family, Apple Cash and person to person payments limits)