I make conscious decisions about my kids' screen time. I know it's not good for them to be sitting still for too long (they are kids, after all) and they need to be active and get outside. They need to have face-to-face contact in addition to instant messaging. And it's bad for their health.
Childhood obesity isn't the only health risk for kids who sit at the computer too much. They could develop carpal tunnel syndrome from overusing the mouse and keyboard, or even computer vision syndrome (a term that didn't exist until the 21st century) from staring at the screen for too long.
When we say parental monitoring, we're usually talking about keeping an eye on what our kids are doing online. But we should really expand the definition to include knowing how long our kids are doing it for.
I'm a big fan of limiting screen time, and not just for the lowered risk of lots of irritating medical conditions for which I would have to take my child to the doctor.
I believe that parental monitoring in this way decreases the chances that my kids will make bad decisions about what they do online. If their screen time is limited, they will use what time they have to communicate with their friends instead of experimenting with chatting up strangers or surfing into a site where they really shouldn't be.
A lot of trouble can come from a bored kid who hops online because he's got nothing better to do. Hopefully by limiting screen time, I can eliminate some of the riskiness inherent in letting my kids online.