Set up another family for a child

If this is a "child" according to Apple's definition (under 13 for many countries), Family Sharing must be set up on the child's account.

You can request the unused accounts be deleted. One way to do this would be to log into the unwanted "child" account and request deletion of the Apple ID. You should have the ID and password from when you first set up the account.

From: How to delete your Apple ID account - How to delete your Apple ID account - Apple Support

Section: Can I delete my child’s account?

"Your child can also request deletion of their Apple ID, which requires your approval.

The approval request will be sent via email to the primary Apple ID email address of your family organizer account. When approving the request from this email, you will go through the process of deleting the child account."

If you have problems doing that then contact Apple about this request: Click here --> Choose your country or region - Official Apple Support Select your country, then a product and work your way down the menu to the desired issue. If you don't see one then try a different product until you reach a telephone or chat help request and ask to be redirected.

To me your main concern about using Family Sharing with a child is making sure you set this on their account: Approve what kids buy with Ask to Buy - Apple Support

You yourself can't. Someone else that is not part of the current Family needs to set up Family ASharing form their devices as you normally would.

The child can then be transferred to that Family Group.

See here to start a Family Group: ➜ Set up Family Sharing - Apple Support

You can only do this to yourself if you have a different Apple account set up under which you could create a separate family group to which you could transfer the child. Note that it isn't normally advised that you have multiple Apple accounts because the system really isn't set up for you to do things that way unless you really have completely separate parts to your life such a business ID and a personal ID.

A different approach would be to ask why do you need to turn off Family Sharing and is there a different solution that addresses that need?