City Dog, Country Frog, which is written by Mo Willems and illustrated by Jon J. Muth, is such a wonderful kid's book. The illustrations are so lovely, and the book is completely moving. Particularly in one scene that always gets me -- you you read, "That was winter," and when you turn the page there is a spread with no words. The silence built into the story there is probably the best part of the whole thing. It's quiet, reflective, and packs kind of a gentle emotional punch in the context of the story. Kids or no, it's worth a read.
(Disclaimer: It is a sad one! With a tinge of happy at the end -- but still sad.)