Amazing Space - This is a great site for young space lovers. At Online Explorations, kids can try our simulations like what would happen if a comet went past a large body like Jupiter or a smaller body like an asteroid. Great simulations include building the Milky Way and measuring light waves. Capture the Cosmos has information on main space topics like black holes, planets, etc. as well as activities designed to help children understand the science of space. There is also an online newspaper and ideas for how to use the site in the classroom or to help with homework.
Best of Hubble - Although not designed for kids, here are some amazing photographs taken by Hubble. Text is written for adults but photos any space lover would appreciate.
Kids Astronomy - This site calls itself "The #1 Astronomy Resource for Children Online" and with good reason. Explanations and activities are geared for young kids and it's a fun and informative site. Categories include; Outer Space (Solar System, Deep Space, Space Exploration, Civilian Space Travel, How Big is the Universe?); Resources (News, Astronomy Dictionary, Teacher's Corner, Free Online Classes); a games section (Puzzles, online and offline coloring, and jokes); and "The Sky Tonight" with current observation information, including a sky map and phases of the moon. The only downside to this site is the ads, which are small but everywhere.
NASA Kids - NASA Kids is an interesting site with lots of information for kids who love Space. Sections include Rockets & Airplanes; Space & Beyond (including a great "Astronomy Picture of the Day" with a brief explanation by a professional astronomer, archives as well); Astronauts & Living in Space; Projects & Games (which in fact has only games, for example "Land on the Moon" or "Mars Jigsaw Puzzle"); a Club House (which is currently unoperational); and "NASA Toons" (informative animations on a variety of Space topics, for example "Meteor Showers"). There is also an Art Gallery with drawings by kids, and a section caleed Stories by NASA Kids which seems to be mostly more artwork (note - my computer has crashed on this Stories link 3 times - I am giving up! :). Overall a good site with great graphics and explanations.
Space Place - This website is chock full of great simulations and activities for young space explorers. Another NASA site, this site promises that science is fun, and it delivers! Try games like an animated trip to Mars where you have to choose 10 items to ensure your survival, print out games, and see a great bunch of simulations, including Cosmic Colors, where you can see a wide variety of heavenly objects in infrared, gamma, X-ray, radio, and visible and ultraviolet light! Good explanations and great effects. There is also a section called Friends Share which describes space projects kids are doing around the world and how to connect to activities going on in your own area. Well worth a visit.