Why not just let the kids look at the damn porn?
If they can't see boobs on the Internet, then they are going to see them in a context that results in the creation of children. It's almost like we're programmed to reproduce, or something...
I work in a rather open office environment now. Our management has no problem if employees blow off some steam by surfing to a site that might not be work related as long as its not excessive (just like they don't punish people for going to the break room to get a candy bar from a machine, or going outside to smoke).
However, click the wrong link and end up with porn (or something otherwise seriously inappropriate for a typical work environment) and you get noticed very, very quickly.