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Celestia is a (free!) 3D space simulation, that enables you to virtually navigate around the solar system and beyond. You can travel at different speeds, zoom in and out and view information about the planets, stars and constellations. There is a ‘tour guide’ for easy navigation, a solar system browser and - a good place to start- an excellent demo (through ‘help’). You could use this in its most basic format to give children some idea of distances, speed and position  of planets in the solar system, or you could use some of the ‘add-ons’ for even more infomation. You can download Celestia from here (use the Kent mirror)