Lil Grips (actually 22mo) really likes Monsters Inc and will sit through the entire thing, but not any of the other animated films we have. We think it's because there's lots of action from the start and there's always lots of cuddly fun creatures on screen even during the dialog, and the ratio of dialog to action is quite favourable.
Several other films we tried didn't work at all. I thought Ratatouille might be good, with the fun rats messing about and stuff, but after watching it through her eyes there's just too much talk and not enough action. Likewise Cars and Up (thought that would be fun with the birds and dogs, but no).
So what else can you suggest?