Which one is more useful?
http://www.nasa.gov/audience/forkids/home/index.html
This website is well organized and contains lots of clear information, but it's very busy.
http://space.about.com/od/educationastronomyspace/a/kidsastronomy.htm
This website has a table of contents, but doesn't contain much more than links.
http://library.thinkquest.org/03oct/01581/SpaceTravelEnglish/main.html
This website is fantastically organized, with clear and comprehensive content.