My Niece turned four in November and likes messing about with my DS. She's useless at using the D-pad and buttons, she doesn't fully comprehend how they work. What she likes doing is playing with the stylus so I would reccomend stylus heavy games. She enjoyed a DS homebrew called Colors, which is a mini paint application, Let's Draw which a pretty decent colouring game of sorts and she likes Super Scribblenaughts as she can just create what she wants. SS is fantastic as it aides her imagination and is a great way for her to improve her vocabulary.