The solution to all of this “think of the children” stuff is that devices owned/used by children should have to be registered as a child’s device, which would enable certain content blockers.

That's kinda the case right now already, but the problem is that adult-only sites don't work with that currently.

So the right solution would be to mandate that e.g. all sites are required to return a header with an age recommendation or something similar, so that a device set to child-mode then can block all these sites. And if a site doesn't set the header, it will also get blocked on child-mode devices

Wouldn't be too hard to do, and accidental overblocking would only occur on child-mode devices, so there's not much of a loss there.

Legislation could then be focussed on mandating that these headers aren't falsely set (e.g. a porn site setting the header to child-friendly).