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.