"police must be able to see all conversations on messaging platforms in order to protect the public, specifically children, from predators."
That sounds reasonable.
Now come back and tell us how you propose to do so that (a) it really is restricted to police and not accessible to some ne'er-do-wells who work out how to crack it in the future, (b) it requires a warrant issued on a genuinely* made serious crime case to a magistrate and (c) by what painful bits of the body you're going to string up those making unauthorised use to spy on political opponents, neighbours, the guy sitting at the next desk or spouses.
* Unlike, say, the warrants obtained on "Nick's" victims.