NextDNS on Mac Restricted Profile

Hi, I'm new to this so please forgive any mistakes ..

With national lockdown & homeschooling a 5 year old, I'm trying to create the safest possible on-line environment.

Our router is now giving NextDNS via DHCP, but on a relatively relaxed profile.

I've created a very strict NextDNS profile to be used for my kids, and it works great for a normal / admin user from an old Mac Mini (Mojave).  But I'd like to combine this with the restricted/parental-controlled user on the Mac Mini to limit the applications that can run.

This restricted user can't launch the VPN profile (presumably some missing permissions).  No information on screen when trying to connect to the VPN, just blanks out.

Is there any way to set the DNS-over-HTTPS/TLS for all users on the Mac, or any other advice for how to get this working?

All help much appreciated :)