Other fun sites:

NASA Spaceplace for kids:  http://spaceplace.nasa.gov/en/kids

International Children's Digital Library: http://www.childrenslibrary.org

National Gallery of Art, kid's page:  : http://www.nga.gov/kids

Internet Public Library KidSpace:  http://www.ipl.org/div/kidspace

National Park Service:  http://www.nps.gov/

National Geographic Society's children's page:  http://kids.nationalgeographic.com/

Sounds of the World's Animals: 

http://www.seaworld.org/Animal-info/sound-library/index.htm