There is a widespread belief that proper technical solutions are enough: give us end-to-end encryption and such, and everything will be all right. But people do things for a reason, and technical solutions are introduced accordingly. We need to look there to understand what's happening.
For an outside observer, the messaging media filter example does not even look convincing. So there's a Bad Guy chatting with a kid who can organize their meeting or take the conversation to a different, non-filtered service. And that's completely okay! (Unless, of course, there happens to be a need to promote Big Brother processing all conversations to protect the kids, heh.) However, when we mention sexual content being sent or received, there's a sudden wild flight of fantasy and countless dangers on the horizon. It makes one wonder whether the real goal was to protect no the kids but the parents, from the worried thoughts that their kid is not completely isolated from sexual sphere. The outcome is that today Beavis can't tell Butthead “Wow, look at those tits!” and send the picture without being reported. What a perfect repressive Victorian childhood, and what an outstanding member of society it will produce!
The taboo is twofold. One the one hand, it creates new positions of power for the people who enforce it legally and in the discourse who won't just dismantle themselves (quite the contrary, see the worldwide practice of drug prohibition and its effects on laws and bureaucratic growth). On the other hand, it creates the inflammatory excitement about the topic in the common person. Media knows well which stories — told from the correct angle, obviously — attract public. As a result, there is a stereotypical image of a maniac hiding in the shadows, and the need to “do something about it”. In fact, maniacs (also a stereotype formed by media, by the way) are rare: in 8-9 out of ten cases of child sexual abuse it's a person close to them who decides to “search for happiness” together in such a way.
If the father makes his daughter send him sexual photos, and he is also the one who gets notified about it, what is the point? Observe the observers, too? That's a bureaucratic dead end. Or is such system, the one to silently look into too numerous parental approvals, already in place? Then what about the father who gets notified about his daughter sending her boyfriend sexual photos while being completely okay with it? Well, maybe such father would disable the feature to stop feeling like a third wheel, but what would be the opinion of Big Brother? Effectively, the point of view of “the whole society” that you could previously silently ignore is transparently codified and enacted by the computer.
