Earlier quoted context omitted.
I like the first part of the idea, which is the header. Heck, even enable it by default. As long as the tracking of the toggle isn't a thing its a perfect compromise. While we're at it, respecting do not track headers would also be nice. This completely leaves it up to the families / parents to control and gives some level of compliance to make the effort worth while. There may even be a way to generate enough noise…
They tried this years ago with ICRA and others. Long gone, but it worked by the webmaster adding metadata to self classify a site. Browsers and other agents could then allow/deny after inspecting the tags. https://sk.sagepub.com/ency/edvol/childmedia/chpt/internet-c...
That's why I am pleased with RTA. Just add one header, done. Most admins and developers could do that in their sleep.