@ Dave Gomm
I'm not sure whether I must applaud or hiss. I feel like lauding you efforts to control your kids' online behavio(u)r, and your resistance against peer pressure. But on the other hand, you seem to be blaming the technological advances, and especially "open operating systems", whatever this might mean for you). The most "open" OS, when it comes to user mangement, is Windows. I suspect you were blaming *NIX-inspired OSes such as Linux or BSDs. Just so that you know, these OSes were *designed* to allow the admins to keep a firm grip on the figurative users' nuts. If you want to tightly control what users can or cannot do with a computer, at any level, these are the best solutions on the market, by a considerable margin.
Anyway, as far as controlling your kids' access to the web and when you know what you're doing, the OS is not really relevant -and if it was, Windows would be the weakest link. I went on to type a whole analysis but halfway through I figured that it was far too much work for dubious benefits, so I'll let you search the web by yourself, you'll know what I mean.