This site was created by the Centers for Disease Control and Prevention, but it is designed for kids 9 to 13 years old. The main page is very colorful and engaging, with lots of flash animation and some really cool cartoon artwork reminiscent of the style that is popular on Saturday morning cartoons right now (that is, it is somewhat inspired by Japanese animation).
Many of the internal links lead to neat little quizzes and games which combine information together with interactive feedback. For instance, there is an animated quiz on peer pressure which asks a series of questions like "is it always better to handle things by yourself or is it okay to get the help of friends?" If you answer right, there is an animated skater that does a little trick. If you answer wrong he falls off the skateboard. I admit I answered a couple of questions wrong just to see what would happen (that's my excuse, anyway), but I thought it was a neat little game nonetheless. The questions on this quiz steered a nice line between "being true to you" while still acknowledging the benefits of being part of a group. Aside from being colorful, the text was written in a clear and casual manner that I think kids will be able to identify with (at least as far as I know).
There was an especially cool section on the site about dealing with stress. In fact, the topic took up several separate pages and was linked together with a nice little animated menu of definitions. I suspect it took someone several months just to put this little area of the website together. In any case, the material managed to combine pre-teen accessibility with a high level of sophistication - going so far as to talk about the different glands and hormones involved in the bodies reactions to stress. This section included a list of 10 tips for dealing with stress (exercise, proper nutrition, different means of expressing yourself, etc.). I think this is an important topic to address with todays kids, who are typically overstressed but who also lack the physical activeness that would help them relieve their stress.
In addition to the material aimed at kids, there is a teacher's corner. This section includes a terrific set of links back to different areas of the main CDC website which are particularly relevant to teachers and kids. It also has some nice material on how to get the kids in your classroom to be active. There is also a "bam bibliography" which contains anywhere from 5 to 20 high quality references for each section in the site - so you know these guys did their homework.
There is a ton of information on this site that would be of interest to kids. They cover everything from disabilities ("How do people who are disabled go to the bathroom"), to bicycle safety.
One thing that is hardly covered at all on this site is information about various forms of illness. For instance, I did a search on the site for "cancer", and I came up with nothing. Now, it could be that kids in this age group aren't really that interested in cancer, or it could be that the authors of the site felt this was a delicate topic to try to address in a general web page like this. Still, kids do have to deal with illness on a pretty frequent basis. Besides getting sick themselves, sometimes they have a sick family member, or sometimes a kid in their class is sick. I could see a real value to giving kids some information about these problems that is aimed at their level rather than forcing them to do a broader search on Google - where a lot of the information will either be above their heads or couched in terms that will make the problem even scarier for them. Anyway, I know this is a tall order, but it's something these folks could aim for in the long term someday.