ADMX applies registry policies, it is the same thing, but different method. It can not be easily changed when CMD/regedit are blocked.
EDIT: But yes. this policy is not supported per profile, at least not on Edge.
Windows and Mac documentation for supported Microsoft Edge Browser policy: Specify URI template of desired DNS-over-HTTPS resolver
learn.microsoft.com
It's simple for kids to change DOH in a browser
Kids could just download a portable browser and use any DNS they want, unless you specifically block it in a firewall or DNS by domains or IPs, everything is allowed.