The U.S. House of Representatives Kid's Page
http://clerkkids.house.gov/index.html

The White House Kid's Page
http://www.whitehouse.gov/kids/

U.S. House of Representatives Educational Resources
http://www.house.gov/house/Educate.shtml

Ben's Guide to U.S. Government for K-12
http://bensguide.gpo.gov/

First Gov for Kids!
http://www.kids.gov/