Mandating that people do face scans or, even worse, send their IDs to potentially scummy websites is batshit crazy though.
If what a government really want to do is ban access to pornographic content then they should be able to make that case to the electorate.
Why not just the parent sets the child device to "kid mode" and that is it. otherwise how do you make different national systems work World wide ?
Your idea is valid but seems much more harder to implement. With my proposal where the parent and the OS vendor collaborate on this seems much more flexible and would work even better, like religious extremists could configure their child and their own devices to block much more then porn.
But of course kids are resourceful and will find a way of accessing content they want to access, for example maybe using a games console or something a parent might not immediately think of as having a browser.
Whether it actually makes sense to protect kids from themselves to that extent I think is a valid question which I'm not sure I have the answer. I know passing around CDs full of such content was common when I was young (in the early days of the internet).
As I said a parent should not allow the child to access social media that is targeting adults. The OS can put a flag and the websites can reject those connections, no need for destroying privacy.
Not all porn is women exploration, there are this days this OnlyFans women that are exploiting men but who should decide what is and what is not exploitation. There is also CGI or AI porn, there are nude and artworks that religious extremists complain about. You can't stop porn so better don't wait for that day and setup your child devices if they need one properly.