Here is a site that one of my graduate students (John Lucas) examined for a class project. During the night he presented, the website would not load (OF COURSE) so he posted the site on his blog and I thought I would share it, too.
John explained: "It would be most useful for teachers that are working with younger students. I think that students could really benefit from the site. I think it's great that it's free and yet it isn't overwhelmed by a lot of advertising. The link can be found at the bottom of this post. I hope it works for you and enjoy.www.kidsknowit.com"