I've found a lot of modern art to be terrific for children: the sizes, shapes, and colors make toddlers stare, laugh, or go "wow."
At the Minneapolis Institute of Arts' Target Wing, there's an intriguing exhibit of Cheryle Melander's and Don Myhre's work. I found Myhre's work particularly interesting, especially the pieces featuring giant white faces. My own toddler liked it, too, but it is a little spooky (for a bit, he thought the three giant faces together were mad at him. Later he thought they were happy. There was another face that appeared to flicker that I think creeped him out a bit too). Overall we enjoyed this particular exhibit. My toddler really enjoys a lot of the modern art in the Target Wing.