Keep them occupied. If they have enough interesting information about things they like, they won't find the need to go look for things they shouldn't see. Iin the book, we have categorized the sites, so kids don't have to use the search engines as much, where they could find potentially offensive material by mistake. I have added a some more links to related information from other sites here. I would suggest that you read these documents with your children, and even print them out to keep next to the computer for ready reference.
Nothing can replace the monitoring of online sessions by parents.
But software can help you keep track of how your children are spending their online time.