When you click the main nav buttons on the left the submenus that appear are not well spaced. Also, I'm not sure about some of the menu choices on the top navigation rollovers. I think Home buttons are more useful without submenus(i.e. When they just get you to the homepage). And if I was a user of the site I don't think I'd think to look under the 'home' rollover to find information about joining the site, or the 'about us' section. As for the other items there, (the login , logout ,register submenus under the home tab) perhaps change the 'please login' at the top to 'login | register' Then when they are logged in this link could change to 'logout'?
There is no sign up link from the 'web pages' page.
I think the overall idea of the navigation is really good though. With a menu at the side. the rollovers along the top, and navigation in the middle of the homepage on load. As I navigated the site, the pages were well laid out, attractive, and quick to load and from a graphic point of view the site looks great. It's a good idea as well, and I'm sure there'll be a lot of demand..
One slight caveat to the above may be that the site looks very grown up. It's a tough job designing a site to attract parents teachers AND kids. I think the balance here might be just very slightly out. Perhaps just a few little touches to communicate visually with the kids(Perhaps one or two of the images on the homepage could be changed). On the other hand maybe I'm talking out of my a@!
Hope that helps though.