@lauren I believe a bloom filter (or trained recognizer) on the video layer will generally be hard to subvert; Lord knows they'll try. That gives a rough cut of "probably porn," and the whine screen can then be passed to an online detector for verification and then follow up (notify parents, ban account, whatever). It's impractical to use that to prevent the image from being seen though, unless we want the kid phone experience to be total ass.
It would also be very expensive battery wide, depending on how frequently it's scanning the whole screen. And "your phone can screenshot what you are up to at any time and send it to the government" feels like it should be pretty viscerally horrifying, even to Brits.
(I don't think this is a good problem to try to solve; my noodling is on whether it's actually unsolvable and under what constraints it becomes so).