This is 2014, and we're in the decade of reboots. This is the reboot of "sit your kids in front of the TV to watch the Children's Channel" thinking. The glowing, phosphorus parent of the 80s, now back with less Big Bird.
Put your kids outside. Don't put them on the bicycle of the Internet; put them on a *real* bicycle. I walked the 1/3 mile to school when I was 6; I could bicycle 1.2 miles in that time, a good 10 minutes walking by myself, well out of sight of my parents. When I was 8, I had a bicycle with a coaster brake, and would disappear outside for hours at a time--by myself, since I had no friends. Sometimes I came back home after the older 5th graders beat the shit out of me for some Freudian satisfaction related to their small penises (too impatient for puberty I guess), I'm sure; but, for all the baseball bats and tennis shoes they applied, they never managed to put a bruise on me, so I made out alright.
This is all a bunch of wanting your kids locked in a room doing a single thing, in a place you know, with the ability to look in and verify they're still doing that one thing and nothing else, so that you don't have to show any concern. My massive internal simulator predicts, via armchair child psychology, that this will not provide a robust set of varied experiences for the child, and so will slow their mental growth and reduce their ability to thrive. History will prove me correct--has proven me correct--but I'm sure nobody will listen and, when it's all well proven that this actually happened, will instead find the next substitute single activity and claim it's different, somehow, and fail to predict the same result.