I wasn't really planning on commenting in this section (I'm not a parent, so who's to listen) but this really needs to be said.
These are some facts of life:
Your children will inevitably be better at using computers than you are
If your children want to find something, they will find it
Internet filters do not work when someone finds out how to work around them.
What is important is to be able to have open and earnest dialogue with your kids and their developing sexuality. In this day and age inappropriate content can be and often is a part of that and needs to be addressed. Whether you want to admit it or not, kids will have questions about sex and it's up to you whether you will be the one to answer them or the internet. You cannot "protect" your kids from inappropriate content, only inform them. inappropriate content will teach your children to "see women as objects and not as human beings" unless you talk to them first. When they are prepared for what they see, then it won't have as bad an effect.
[edit] and don't make the mistake of thinking this only applies to boys. This sort of stuff can have just as much an effect on girls. Girls mature faster than boys and this includes their sexuality. You need to be able to talk to them as well.