I think if you as a parent have router controls and block adult content on their mobile plan if they have one (which I have seen as an option), then you are already doing a lot.
Most routers from ISPs come with "adult" content filtering enabled by default I think, at least the ones I've had have had this on.
VPNs already work and I can't see them not working, so that's always an option I guess, but they are also still an option with ID laws (ie connect to a region where they have no such laws).
Children's safety online can't involve limiting access and tracking everyone who ever goes online with their national ID attatched to every request (basically).
I think it'd be better if we explored the option that involves a parent blocking websites either on your network or on a device they give to you.
Hm I know it's common for me. If it isn't in canada or elsewhere, then that's just lazyness and a lack of care by your isp/router manufacturer (which makes sense cuz theres a monopoly on internet over there right? (Rip)).
Anywho, adult filter blocks on routers are a really easy thing to implement. If it isn't then they simply don't care to help explore the simplest of options for parents restricting access to bad sites.
I've heard of apps you can get for child devices that do a similar thing and let parents track their kids, which might be better anyways, assuming they arent a privacy nightmare (if parents dont prefer buying a smarter router).