How do I change a user profile to a child safety profile

I can't answer your question directly as this isn't something I've looked into. But just a few thoughts which worry me about how useful and effective child safety is..

Caveat: With the new Creator's build, some of the following may now be out of date- I have no way of knowing.

Here's setting one up - in the USA (note- federal law and the credit card):
Adding a child user account in Windows 10 | PCWorld
That may explain why swapping may not be easy.

Note this peculiarity:
Windows 10 How to set up a Childs user account - Microsoft Community
Family Safety child can change from Microsoft - Microsoft Community

Checking one:
https://support.microsoft.com/en-gb/...-after-upgrade

You would be best advised to set one up from scratch as a new user, and consider whether they should log in with a live id or as a local user. (How you have set up the existing account has a bearing on this as well).

Go here:
Windows 10 Tutorial Index Windows 10 Tutorials
and search for
child
and you will find a number of tutorials related to this such as
Child Account - Add or Remove from Your Family in Windows 10 Windows 10 User Accounts Tutorials