This is where it gets tough really fast. For arguemnt's sake, let's say semantic analysis can properly identify patterns well at 95% level (oooooffff already can see problems).
Public/wall comments on any platform are relatively easy to police. But, I argue the vast majority of problematic grooming comes from DMs. Policing DMs seems worse than trying to do hash analysis of private photos.
I don't think there's a win scenario here for anyone involved.
[edit]
> they could check the birthday of people,
yeah because this is answered honestly by predators.