Pre-K to 4 Science Sites:
Nasa for Kids: Games, facts, and more fun stuff about space.  Can't go wrong here, folks.  
Lawrence Hall of Science (for kids): Play online games, download activities to do at home and more. 
Smithsonian for Kids: Fun online learning for kids from the Smithson. 
Earthquakes for Kids: This is a great site from the USGS that lets you learn all about earthquakes.