"Think of the children; make sure the government can easily read everyone's text messages" probably shouldn't.
I can’t find a con to protecting the children. But what’s doubtful is whether you actually are protecting the children with this policy.
I think that it would not.
This is the result of government taking over the protection of specific children:
https://www.kansascity.com/news/special-reports/article23820...
If the government has any intention to protect children, and intention at all, this is where they'd start. These kids really need it, and the government has all the control they could possibly want or need to do it. They not only utterly fail, they regularly cut funding to what little efforts exist to help these children.
So no, government does the opposite. Therefore their intention cannot be to protect children.
So what OTHER effects does this have? Well it greatly increases the government's ability to directly interfere in the lives of citizens. Which they, not seeing any irony, use to send more kids to their foster care -> juvie -> prison carrousel. In fact that this enables them to do more of that, is one of the main reasons to do this. Seriously.
When there are studies that show that, as a kid (on average, discarding extremes), you're better off abused at home than "taken care of" by the state. On average, you're better off without government help. For example:
https://sci-hub.se/10.1257/aer.97.5.1583
(I do not deny that there are situations where state help is necessary. However, anyone that knows anything about the foster system knows that they don't help in such situations. These situations are too difficult/too much violence to deal with/no expected positive result/... and explicitly target kids with small or nonexistent problems because they're paid per child, based on the care provided. So 24/7 care for a problem-free toddler is where the financial incentives are for them. This even applies to medical problems: child has cancer? You're entirely safe from youth services. Why? Because caring for that child is too expensive ... the EXACT opposite you'd want to see them do, but, of course, financially it makes total sense. Ironically this means effectively random (poor) kids go to foster care, then juvie, then back home, because it's a financial disaster for foster care, institutional or otherwise, to take them back, so suddenly there's "no more need" for care. It's a financial disaster for the parents too, of course)
In fact there are studies that show that social workers REFUSING help to children (in reality to parents and schools) works very therapeutically, and actually fixes problems, by (amongst other things) creating a great need for the problem to get solved, rather than taking away the child, which of course takes away the problem without solving it, doing incredible permanent damage to kids in the process.
So, in THEORY they do this to protect children.
In PRACTICE they do this to damage children, and to do more damage to more children.
If some murderer says they killed their victim out of love, the conclusion isn’t that love is a bad motivation for doing things. Or to say that in theory love is good, but in practice it’s bad.
I even abstract from both reasons of why the intent might be lacking. Are they outright evil? For example, are they using this as a deception to either push their own projects or even to sell real estate to these government efforts? Are lawmakers amoral? In the sense that they utterly don't care what they do, just how it'll be received. Or are they dumb/deluded? I imagine it's a combination of all 3 in reality.
It's just that maybe some of the lawmakers involved are deluded about what this does, but that's the most charitable interpretation possible.
You don’t think that people aren’t on board with things that do actually benefit children!?
Stop making consolations. Math doesn't care about warrants. There is no higher oversight than 256-bit symmetric ciphers.
So the trade-off is that law enforcement have to look for other ways to enforce the laws - like they need to find one side of the message conversation and get a warrant to access the data stored on their phone to view the communications. Technology has actually given them far, far more scope for access into people's lives for surveillance than they've ever had! They are exaggerating the problems with things like end-to-end encryption because it's taking away a very effortless source of surveillance - something that they probably shouldn't have ever had (the ability to easily snoop on everybody's phone calls and messages, which we know in the US and other places that they did warrantlessly for many years).