How do I transfer or delete my kids' iCloud account?

See this article about how to move a child to another group using Family Sharing - How to transfer a child to another Family Sharing group - Apple Support

As for deleting a child's account, 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.

Another way would be for the child to make the request.

From: How to delete your Apple ID account - How to delete your Apple 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 the child's account password is unknown, see the section in this document about using a family Organizer's device to reset the password --> Change your Apple Account password - Apple Support

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.

How old are the children? If they are under 13, you can only transfer them to another Family account.

You can see the instructions for doing that and for deleting the accounts here:

How to transfer a child to another Family Sharing group - Apple Support

Can you explain what you are trying to do? Remember, you can’t include sensitive content in any messages or screenshots. example: instead of “I want to move John to his own account” say “I what to move Kid_1 to their own account.”