Choosito. While Choosito isn't 100% free, there are free components that can be used by anyone with an account.
I spent a little time investigating the site and was pleased with what I learned. I did a search for something generic and got a lot of results. I modified my searches for different reading levels and received a lot of different levels of results. It was exactly as anticipated.
I think this is a good source to refer students to use when they need research. It's got a better response than google as there were no advertising coming back in the searches, and with google, it seems that those results are the most common.
Because of the leveled results, I think the site will be helpful for kids of all levels when it comes to research, despite the age of the students. Sometimes in researching, students find materials that are so above their level that analyzing them and using them for a successful paper is often a challenge. This site helps alleviate that problem.