How do I delete a child's account without a password

My child created an iCloud email/account and doesn't have the password. We created a new account for them and want to delete the old one so that it isn't registered under the family anymore.

I've read Move a child to another group using Family Sharing - Apple Support and How to delete your Apple Account - Apple Support but have no luck since we don't know what the password is and don't have an email set up to reset the password.

This account isn't attached to any devices and we just want to remove it from our family group. Thank you for the help in advance.