Primary Games Arena - School Curriculum Games
Don't think we've highlighted this set of games for children. They're Flash based, and cover all the subjects. You can select by grades: 1-6. Also, a few teachers games, like Captain Broadband, designed to guide teachers on Internet safety.
If you have a way to access the journal Computers in Schools, Google Scholar indicates that several articles mention it. Would be great to see some paralytics on these things.