So if we secure personal devices of children, with simple, standardized "child-owned" marker, we're basically back to 80s/90s, where children could occasionally get access to adult material via friends or irresponsible adults.

In my opinion that's more than enough, especially when you compare it to requiring everyone to identify themselves. It may be ZPK on the tin, but likely it will be close-source, corporation owned implementation, which will have holes. Then in a few years we will learn that Meta exploited them for years to sell your soul for ad money.

Btw - students occasionally steal teacher's cars. Should we block engine start with ID check too?