Backdooring mainstream encryption will substantially decrease security for criminals. What is often brushed over by proponents is that it will weaken security for all of us.
Because there are criminals in the government.
This is an easy problem to solve with a proper whistleblower program. There are people adjacent to these criminals and they may be dependent or fearful of repercussions for coming forward. Treat them with witness protection and appropriate and large (millions) in monetary incentives. Like most things in life, you get what you incentivize. But this issue is about total surveillance not about protecting children.
In several countries (Brazil, where I am from included), the law is that possession of child porn, for ANY reason, is a crime.
It means that trying to give evidence to the police, will get you in jail, even if the police doesn't want to arrest you, they have to, because that is what the law says.
Things get worse in certain countries that went along US child porn laws and ignored local culture and laws regarding related areas (for example, in Brazil age of consent is 14, and in some Brazillian regions marriage at that age still makes sense, usually due to harsh geography of the area that make it necessary, cue lots of young couples surprised when their personal photos are considered a crime...)
Also: law enforcement assumes criminals and others are stupid and cannot not apply encryption and information hiding themselves. Plus creating your own chat app is extremely easy, so why even rely on the big and obvious ones, just DIY.