NASA's kids site, with pages on air, natural hazards, land, water, and how people affect the earth; multimedia "adventures", games and activities.
Another site from NASA, this time on the exploration of our solar system
A portal to the world of infrared and and other wavelengths
A project put out by the University of Utah
This is the site of the Hubble telescope, and it's presented beautifully, with lots of exciting images. The information is presented simply. There are several sections, including one of "Fun & Games". A great one to interest a child in astronomy and science.
Another great site about astronomy and science. It's funded by NASA but seems to be produced by people at the University of Michigan. It has more information than the Hubble site, and also has a Fun & Games section. It aims to be a resource for students as well as the general public, so the information is presented at a suitable level. Indeed, you can pick your level - Beginner, Intermediate, and Advanced.
A very comprehensive site for elementary / primary children. The texts are available at a range of reading levels, and in Spanish; and there are quizzes on each section of the text (look under "activities").