I have never seen a better book for explaining evolution basics to children. And this is the kind of thing we need to do a lot more of- getting the younger generations into this fascinating science as early as possible, that they might own the wonder and beauty of evolution.
The book is a bit dated, being eight years old now, as we have discovered numerous fossils since the publishing, where in the book it refers to long segments we still don't know. The Mesonychids are portrayed as pawed rather than hoofed in the illustrations. But it doesn't matter- this type of book is so rare, so needed, that even dating doesn't detract from it's groundbreaking presence. And though it comes across as a bit of a "Just So Story", it is an entirely age-appropriate approach. The adult reading this book should simply explain to their children the extra bits that we have learned since it's publication.
The drawings are beautiful enough to entice the pre-reader into wanting to learn more, and see more. Ted Lewin is one of my favorite children's illustrators, having previously done the beautiful Day of Ahmed's Secret and The Storytellers. Your children will be able to see step by step how whales changed. A fourth-grade reader should be able to get through most of this book, and may want to ask their parents for a few of the words they don't understand. But this is also good, for it encourages them to ask more questions about evolution and pursue understanding.
McNulty picked a wonderful example for her book. For a long time, ever since Darwin, evolutionists were made fun of by the Literal Creationists for proposing that whales came from land animals. It seemed like the right idea, but truthfully the evidence was sparse. Yet biologists remained committed to the method, and to expecting the parallels from other organisms to bear out in future fossils. And has happened time after time, when evolution makes predictions, the fossils appear. And today, the Cetacean fossil line is one of the most articulated and defined fossil lines we know of.
Your child will not only learn about evolution, and learn to love it, but he or she will be prepared to know the basics in defending it, considering evolution as simply a second skin, to be cherished as part of themselves.