Seems like it would be really hard. A TV channel or studio puts out dozens of shows at a time. Something like Youtube gets its content from its users which upload gazillions of videos a day. Content would have to be rated by AI. They already make you sign in if it's flagged as adult content. Google has option to restrict adult images in search, again very likely done by AI.
I don't see this as a massive problem. Anything on the web illegal, like people planning terrorism, would probably be watched by authorities anyways, again with help of AI.
Sure it's a massive problem, but one that needs to be addressed. There didn't used to be movie ratings either.
Kids used to hide X-rated magazines under their mattresses too. Nothing is perfect.
Taking another tack ... I think we need to get much more aggressive about monitoring and policing online propaganda that promotes hatred or violence against groups of people ... as I believe we already do with importing books, etc ... but that's only the law in Canada vs the world wide web.
Parents can monitor a child's browsing history, regardless of what room the child is in. Reviews/ratings of websites would be helpful in that.