Thank you for these suggestions, I wasn't aware of Truple or CleanBrowsing and will look into them.A couple of options I've tried that have worked ok.
Truple.io have a free filtering app on Android though I think their filter on iOS is a paid for service.
CleanBrowsing.org free DNS filtering is another one. Again, my experience is with Android but have found the adult filter DNS settings work pretty well.
Worth noting though that DNS settings can be changed if you know that's where the filter is, so simple manual settings may not work unless you can ensure access to settings is blocked. If it's a dedicated app doing the filtering you will probably find it has some kind of password protection to stop deletion. If using manual settings it may be you'll need to find an app locker app in order to block settings access - if the phone you're using doesn't have this function in parental controls.
Until now I have used Cloudflare's option but today I discovered that our 12-yearold could access for instance "xvideos". How that is possible I just don't understand. I have:
the Cloudflare app set to to activate adult filtering (1.1.1.3 etc)
and
Screen Time set to limit adult sites
and
Screen Time's age limit of apps set to "9+"
I have also noticed that even if I prohibit YouTube's various addresses they still can be accessed.
This is incessantly frustrating.
I am seriously considering revoking phone privileges here and give our child a dumb phone.