Going along with what Jonez said, I'd recommend any humongous entertainment game for the younger audience. Sure it's almost two decades old but it made up a lot of my childhood.
I believe Fatty Bear in particular was designed for toddlers around that age, but most of the games are great for a small kid.
For something a little more educational, The Learning Company's Reader Rabbit series also has games for 3-4 year olds iirc.
If you're looking for regular games with simplified plot/gameplay, why not hand the kid some nintendo thangs? : D /shot