In general, I am a hugely pro-choice. If the kids think they are up for that kind of websites, let them be. The odds that they see something they'd rather not have seen are a thousand to one. Sure, not impossible and there will be plenty of stories, but it's not worth it to censor the internet like this.
Better talk with your children what the dangers are. Tell them how burglars find that you are on holidays and then break in, or how your Facebook images could be abused by that bully to make you look stupid. It'll make them be more careful, and you needn't use securly to block Facebook's photo upload feature. And yeah, you can also take the lead on talking about sex, better to have them hear it from a trusted source first hand. (I appreciated my mom's effort to raise me, let's say, aware of how stuff works.)
Also, asking people to do something rather than demanding or prohibiting, works always better[1]. If you tell your kids they shouldn't do something, they're more likely to experiment but decide it's better not to, than when you prohibit it and they're going to try and find holes. And believe me, try they will, often either finding a way or having someone fix it for them.
Lastly, I know someone who had this kind of block on his pc. Dutch kids, and I guess any, aren't frugal about swear words like "fuck". The kid who had this filter installed couldn't see a significant part of the forum since it blocks the entire page over that word, and he missed parts of discussions and conversations. Then someone tried to be funny and simply put "fuck" in 1pt white text in his signature, almost entirely blocking him from the forum. In the end of course, he found a way around the block. There are always ways.
I know this is your business and you're not going to stop for this kind of personal morals, but I wanted this to be out there. It's certainly nothing personal!
[1] http://lesswrong.com/lw/4e/cached_selves/ 9th paragraph.