Identify devices, not people.

Distinguishing between child-locked and unlocked devices is something any website should be able to do easily. Adult-only should be a config setting.

Vendors shouldn't sell unlocked devices to kids.

Then it's up to parents take sure their kids only have locked devices. (Or not, if they're okay with it.)

What for? I use family link for my kids devices. It works good enough. Everything else seems way too intrusive.

Apple is horrible in this regard. Their solutions never really work.

A joint venture for an (optional) cross-platform family app would be more than enough. This, plus a (voluntary) content rating that's offered via an API (could even be simple meta data on a webpage). Done.