I don't understand why all teh child safety systems require age verification. Why not have a single setting on a smartphone that sends a 'child' flag to every single app or website, which then reacts accordingly? As long as you ensure that the browser can't be changed or modifed, it should be fine.

The California law works this way, and it doesn't even have to require the browser can't be modified

Then adults could lie about their age and benefit from the data protection laws only granted to children for some reason.