You might try "The Kids' Book of the Night Sky" by Love, Drake and Collins. That one has been quite popular here at various points with two of mine. There's a preview at Google books (which I can't seem to link since I am hopelessly lame at such things).
Maybe Couper & Henbest's "How the Universe Works"? It has lots of fun experiments--it's out of print, but lots of used copies floating around (on abebooks, for instance)--it's either DK or Readers' Digest, depending if you get the UK or US edition. My youngest likes this one quite a bit.
peace
minnie
