National Geographic

National Geographic Explorer (simple) http://magma.nationalgeographic.com/ngexplorer/pioneer/

National Geographic Explorer  (harder) http://magma.nationalgeographic.com/ngexplorer/
archives: http://magma.nationalgeographic.com/ngexplorer/archive/


National Geographic Young Explorer (very simple) http://www.nationalgeographic.com/ngyoungexplorer/

National Geographic Extreme Explorer (harder) http://www.nationalgeographic.com/ngextremeexplorer/
Free resources by National Geographic for kids
http://prometheanplanet.com/en/store/publishers/national-geographic/national-geographic-interactive-whiteboard-content.aspx