It's Probably Penny, by Loreen Leedy

Lisa and Penny are back. This time the assignment concerns prediction and probability: what might happen, what is impossible, and what are equal and unequal chances. Will Penny want to go on a walk, bark at a squirrel, or see a shark? And who probably ate the missing peanut butter and jelly sandwich? Here's another mathematical topic that lends itself well to a picture book and the antics of Penny.