Teach Your Children ...
... Do not rely on technology.
In the course of my work I often get confronted with the issue of people having unwanted items such as viruses, spyware or porn on their computers. Often the biggest question they have is, "Where did this come from?" There are so many different sources in this age, that it is nearly impossible to answer that question. So instead I will tell you a story about a person who knew the answer. (Excuse the vagueness below, I am writing this to not give away any hints regarding the identity of the family or any member thereof.)
This person was a conscientious parent. They have up-to-date anti-malware protection that does real time scanning as well as a content filter application that does constant monitoring of internet traffic that is set to block adult sites. They use iGoogle and have the default image search filter set to "SafeSearch Strict". Most people would think this is sufficient! Now the kicker: this persons' child did a search on "w_____ and j______ c____" on Google images, and on the first page received multiple nude images. As many children do when confronted with something new, they clicked on an image that was of a type they had never seen before. This took them to another page that had hardcore pornography. At this point the child went and told the parent what had happened. Seeing what was on the screen convinced the parent their protection was not enough, but still being a skeptic, the parent did what the child said they had done. It worked, no alerts from the content filter or malware protection.
The moral of the story, "Teach your children, do not rely on technology."