Go Diego Go Safari Rescue is a fun and (slightly) educational game for the DS. It’s for the youngest players, and as such, is very easy. However, the designers did a good job of making most of the activities engaging and educational.
I have two little cousins, seven and four, who always want to play my videogames but they can't get a hang of the controls or concepts to most of them. Go, Diego, Go! Safari Rescue is perfect for them.
The game isn't terribly long, and is easily broken in most levels in the sense that it's actually impossible to lose. This may be a frustrating concept to parents as there are no consequences for failing, but read into it how you will.