Both the boys and girls I know all seem to have their own DS system!
As for educataional games, these are the ones I can remember right now, don't want to go digging through the game folder to find them
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I did it Mum, Spelling! was one of the games my son loved at 6, along with I did it Mum, Reading!
Junior Brain Trainer was ok, but not that good overall. Big Brain Academy was a similar one, that I thought was a bit better.
Easy Piano, which comes with a little piano add-on, has been teaching my son to play the piano.
Scribblenauts and Super Scribblenauts are both brilliant, according to my son, but might need to wait until your son's spelling ability is reasonably develped, along with lateral thinking as it's all about finding ways to solve problems.