I have a feeling that, currently, the cons outweigh the pros. Maybe if the digital landscape changes in the future, then we can reconsider those limitations. So far, the mobile Internet - the apps mainly - is a hostile environment for a young human, and I'd say they are pretty dangerous to the adults too.
Who? The lawmakers? I believe banning children up to a certain age from having Internet-enabled mobile devices is a good starting point, and both child protection services and law enforcement services should fully cooperate with parents in enforcing that ban. If you see a kid smoking a cigarette or taking drugs, you know parents have failed somewhere along the road and it would be preferable if the government could step in to help.