For one, the kind of psychotic violent criminals that might assault random children in the streets are unlikely to get caught thanks specifically to cyber-surveillance.

I am perfectly comfortable with having my children run around in the same streets as people selling drugs or stolen credit card numbers online.

But to address your point less literally - "it might make it easier to catch criminals" is an extraordinarily weak justification for compromising the privacy of BILLIONS of citizens (WhatsApp, sadly, runs most of the personal communications in vast swathes of the world).