We limit device time and I try to be as involved as I can, we ask them what they're doing, we try to play games with them, etc. Every day. We keep repeating the mantra about not talking to strangers online, and to tell us if they talk to you. Honestly, I think it's other stuff that will make the difference, spending time with them every day, talking to them about everything, and encouraging active participation in real world stuff like sports and some volunteer work for the homeless through our church.
I don't disagree with the "no internet" idea exactly but at some point they'll be exposed to it and the way things are, I wouldn't be the least bit surprised if a smartphone was required equipment for junior high or highschool within the next 5-10 years. Already our schools send us just about everything online, we have regular emails with the teachers and they use google docs and other platforms for online sharing within the school.