I find it curious that society views mind-altering drugs as highly dangerous, yet doesn't see the same risks associated with purely behavioral addictive cycles. There are many ways to trap a human animal in a dopamine loop, and at some level it's hard to distinguish a deep MMO habit from a heroin addiction.
The genie is out of the bottle: there are more superstimuli [1] in the world than ever. What's most frightening is that even if you create the perfect household dynamic, kids' expectations will be shifted based on what their peers experience. At some level, a tradeoff is made between a healthy relationship with technology, and not ostracizing a child from their peers. There is an age past which it is social death to not have a Facebook account or your own phone, and it keeps shrinking.
When I have kids, part of my strategy will be to have as many intrinsic limitations as possible: for instance, they can spend all the time they like with a non-networked Raspberry Pi, or an offline eBook reader, or a tablet that does nothing besides Wikipedia. I also like the idea of a single family computer in a shared/public space, and time limits will be as much about balance between family members as about trying to enforce healthy habits, at least in the cognitive framing.
But, like most people who don't have kids yet, I'm sure I'm mostly talking out of my ass. :)
[1] http://en.wikipedia.org/wiki/Supernormal_stimulus