The guidelines of Facebook oughta be the same as the guidelines of hn which oughta be the same as the guidelines of kindergarden. Don't post anything you'd be embarassed if anyone else saw and don't say anything online you wouldn't say in person.
But it looks like Facebook is trying to grow past it's original niche into the online identity provider for all. Which means someone else will have to fill the void to give kids a chance to discuss sex, drugs, and whatever other startling discoveries they seem to make on a daily basis.
Those crazy kids are a fickle bunch, and throughout history they've wanted to not be where their parents hang out.
But with Facebook the two groups are separate. Think of it as a house: It's all one place, but each group has their privacy. The parents can have a dinner party upstairs while the kids get drunk and party in the basement.
You have your friends and you have your family. You want your granny to be able to see the pictures your mum took, but you don't want your mum to know you are friends with bobby that she disapproves of.