By the time it makes sense for kids to be using social networks at all they may as well be using the real thing. Once it makes sense for them to use social networks they definitely aren't going to use the one Mom or Dad can peer into (and everyone else's mom or dad in the group).
If you are worried about the implications of social networks for your child then don't let them use them at all, because it is next to impossible to create a safe space online. Being safe online is more of a learned skill, than an environment you can create, and by the time you should be let to wander the web at all you should probably be old enough to be taught those skills anyway.