I would not freak out that much (and you probably don't). Try to look at it more realistically. They probably gonna pick up on these things anyway, so better if it's within your own family. On the other hand, if you don't want any kids to see any kind of stuff, then don't let them. But don't make a big fuss about it because that only makes them more curious.
It's like making a hole in a wall and put up a sign above it. "Do not look through the hole". What do you think happens?
BTW, I do agree that children should not be allowed on these social networks. Not so much what they can pick up or see, I'm more worried about those scary people who "like" kids (you know what I mean right?). That is more disturbing for me. Fortuantely NL and US are working together also to get these idiots behind bars. I think it was last week they arrested a few which were using tor sites (not bit torrent but
tor)
In in NL you have Hyves. It's similar to Facebook. I think 75% or more are underaged kids. I have warned kids (and parents) I know many times when they post there BB pin, phone numbers and address details. Parents should know kids are that naive.