HN user
You're right. Defining the criteria for marking an app as harmful might not be straightforward or easy. But we're talking about the education and life of the children. These difficulties shouldn't a preventing factor. Maybe it's easier to allow services and apps one by one when there is a need only. Whatever that's not, let kids have a phone with internet and everything that comes along.
I've never lived in a country where carrying gun is allowed and im not much familiar with the topic, so I won't comment on that. But I lived through the time that there was no phone and then when there were phones with non of these junk apps on them and now. This, I believe, makes it very clear, what the real cause of these problems is.
It's not phones or tablets. It's the wrong doing apps / social netwrks / games. Same applies to friends: there are friend circles that bring only bad behaviors. We don't ban friendship because of that. Ban social network apps. Ban addictive games. Ban notifications and any other FOMO-triggering mechanisms. Don't ban phones.