Did you guys read the letter by chance? They aren't calling for Apple to be a nanny at all, honestly. They are simply requesting that Apple provides tools to assist parents in this. They also cite strong evidence that shows some of the struggles parents are having with this considering how active technology is in the classroom. They mention how it's unrealistic to assume parents can fight this alone. I agree that it's ultimately the parents responsibility (and the letter says the same thing). But I see no issue with Apple offering tools to assist in this. Parenting now is vastly different than parenting when I was a child. And unfortunately, there are a lot of parents who are parenting in a generation of technology vastly different than when they were a child. So it's reasonable to assume they may seek a little assistance with this.
The educational sector seems to have a solid approach to this. There are MDM platforms that really allow you to limit quite a bit with devices. So in 1:to:1 environments, teachers can have real fine grained access to the devices they deploy and have better control over what their students are doing. Having controls like that for parents wouldn't be the worst thing in the world.
The letter mentions how an "All or nothing" approach can have negative consequences, and that's really all that's available right now.