I hate to say it but those internet blocking software are quite easy to get around. I found out how to get past those things before I had/knew how to access an email account, in the days before I had an internet connection or felt safe using a computer. I learned how to do that when I right when I entered junior high, I never had to use that knowledge myself but I sure did tell other people how to get around it.
It was simple stuff to, like using google image search since some bad blockers allowed use of it to find porn. If that didn't work we instructed people to search for it in a forigne language, go to not so popular websites such as geocities or where people start their first websites, sign up for it on email, etc. And keep in mind I learned this before I even knew how to use email or use the internet, people had to walk me through it step by step when we used it in class back than. (and chances are if you reading slashdot your either old enough for this kind of stuff or already know better ways around this kind of software, so I don't feel bad posting this)
So the best bet not to relay on software to teach your kids, and to just talk to them. And chances are if they want this stuff they can find it no matter what you do. After all nothing is really stopping them from buying the stuff at an Adult store, except the oddball cashier who may actually care their underage.