TweetGo back to the screensaver settings. Next to where you assign the time you will see a check box for "On resume, password protect."
But anyways that won't really keep your kids off. What you need to do is buy some parental guard software and set up a user account just for the kids and have only certain sites accessible like wikipedia.org and dictionary.com so they can do their school work. Find out what site they are downloading from and have the site blocked.