I don't see it as an issue. The bad part of social networks is feeding you never ending crap. If kids want to talk to each other... what's the issue with that? We had phone calls and party lines and passing notes before. (Or letters for those who were into that) They were as much a social network as Google doc comments.
It's the feed that allows them access to public posts that is a big problem. Let's start with children accounts where the feed is restricted to friends / followed accounts and only in chronological order.
As I type this, I realize this is exactly what a lot of us have been saying should be done for adults too. And why is it that all social media either moves towards this nebulous endless feed or actively pushes it. Very telling.
Point being, humans are highly adaptive creatures. You have to cut the source.