We have installed and used Celestia (the free space simulator) an amazing tool to explore universe and show to kids what it means exactly. It is fun to explore the universe using it along with the kids and it's add-on.
NASA and ESA have used Celestia in their educational and outreach programs, as well as for interfacing to trajectory analysis software. (Source: http://en.wikipedia.org/wiki/Celestia)
Official Website: http://126.96.36.199/celestia/
Add Ons: http://www.celestiamotherlode.net/ & http://celestiaaddons.wix.com/celestiaaddons