Another day, another nonsense "think of the children" line
This isn't about finding abusers, or child pornographers. That's just the usual excuse. The only use for this sort of thing is mass surveillance. No more, no less.
Actual criminals, the ones out there hurting people, would use something else. As I've said before, there are apps that allow strong encryption that only the end users have - as they generate them when they launch the app. The code for such apps is already online and downloadable. Takes a fairly small amount of time to get your own personal chat network running.
If it truly were about the children, then end to end encryption would be a requirement of all online tools. Remember the old hole in Yahoo Messenger? Allowed people to connect to running sessions and see the users' cameras. If the encryption in place across the net now gets weakened by back doors, then that is what we'll be going back to.