The vast majority of kids would be stopped by the OS account restriction, especially now that they’re growing up with iPhones instead of Commodore 64s.

If I were building the feature, I’d put the “adult” header behind a password.

But that’s a solution only for the one browser. You’d need to forbid the OS account in use from installing new apps so the kid can’t get a different browser. And now it’s starting to be inconvenient for the parent.