The Red Fish - a fun site with some language activities and some just for playing with the computer! http://www.poissonrouge.com
National Geographic for Kids http://www.nationalgeographic.com/kids/
PBS Kids - so many great activities to practice English. http://www.pbskids.org
TeleTubbies have this site with many acitivies children in England play too. http://www.bbc.co.uk/cbeebies/teletubbies
Interesting Things for ESL Students is a web site with many English language games and practice activities. There are many ways to practice English vocabulary and grammar, from beginning level to intermediate level. http://www.manythings.org