If we must have something, I’m in favor of the OS-level flag where a parent can set the device as belonging to a minor and the apps and browser can receive the flag. Lock it behind the most basic parental controls that can be protected and remotely monitored. There are some loopholes with kids downloading apps that allow custom web requests without the flag being sent, so there probably needs to be an app allowlist or special App Store category to match.
Carve out some legal exemptions for Linux distributions that no kids are using and leave it to the iOS, Android, Windows, and Macs of the world. Any parent who doesn’t like the idea doesn’t have to enable it on their kids’ devices.