If something like this was in widespread use it would have much more impact since countries would see whole swathes of the internet immediately go dark when they make stupid laws.
It's an interesting idea. I presume that the there would be similar laws to selling guns. So there would need to be the national ID card and checks when selling any internet-enabled device. TVs, phones, cameras etc.
I as, a parent would probably need a phone safe, into which I could place my phone when I wasn't using it (though I suppose conceal-carry would be permissible). I;d probably want to have biometric locks on my TV, Chromecast etc etc and the children wouldn't be able to use the TV unsupervised unless all smart functions were locked down.
Doesn't sound particularly cool.
Phones can have passcodes, fingerprint readers, facial recognition (for parents face) to keep kids off them.
Devices can have multiple user accounts, each with different purposes and applications. On my linux laptop, I have two accounts, one for work & one for personal, with distinct applications and configuration.
If all else fail, each manufacturer can product a simple device that can only chat & call with parents in case of emergencies. Can be a simple smart watch or pager like design, or just a dumb phone.
We are at the point where children should not even be exposed to the news (which is primarily incendiary politics these days) unless it is a major event. Smart TV's has so much garbage on them, why should they be allowed to even watch what they want on it?
Either way, ALL of these requires the parents to actually be parents. We can create the perfect technological solution but if the parents expose the child to porn/drugs/social media etc etc and fry their brains, it is a parental problem and not a tech problem.