My boys have just turned 3 and 5 and love
Hungry Hippos
Loopin Louie
Piranha Panic
Don't Wake Dad
The simpler the game the better I think at this age - and nothing too fiddly.
We got DS1 Mousetrap for Christmas last year - big mistake, so hard to set up (not what you want on Christmas morning!) - we've just put it up in the attic until they're old enough to set it up themselves!