The following search engines have all been tested and are safe for elementary students.

Ask Kids
http://www.askkids.com/

Fact Monster

http://www.factmonster.com/

KidsClick!
http://www.kidsclick.org/

ALA Great Web Site for Kids
http://www.ala.org/greatsites

Awesome Library
http://www.awesomelibrary.org/

Diddabdoo
http://www.dibdabdoo.com/

Kids Search Tools
http://www.rcls.org/ksearch.htm