This is an excellent introductory book on meditation that we recently used with great success to teach children to meditate at a Buddhist summer camp. The theme of the book was just about right for kids aged 4-9. I did not find the material "dumbed down" for this age group as a previous reviewer suggested, but perhaps older children might find it to be so. The piggies are cute and kind of cool (I liked them!).
The book guides children through the reasons why meditation is necessary, and believe me when I say it is the best book on the market for doing so (I looked around extensively). Especially good is the "mind-in-a-jar" experiment at the end of the book which gives a concrete, hands-on demonstration of why meditation is necessary. We placed dirt into a jar, spun it around, and the kids noticed how the sediment settled to the bottom after they meditated. A note, however: a little dirt goes a long way - don't put too much in or the experiment won't be as successful.
Some practical notes: it is best if this book is taught by someone who is experienced in meditation, as is the case with all meditative traditions. Furthermore, most children can't really meditate effectively for more than five or ten minutes due to modern attention spans. Just like adults, some children can meditate in full-lotus, some in half-lotus, and some can't do either. Especially if teaching in a group, care needs to be given to all three groups and children should be allowed to do what is comfortable. Finally, instruction is needed on the importance of giving ourselves some time to "switch modes" from meditation-mode to running-around-and-playing-mode in order to adjust. If children just get up and run around one second after meditating, the sudden switch between sympathetic and parasympathetic systems may cause some discomfort.
Overall, I believe this is a "must-have" book for anyone who wants to teach meditation to children aged 4-9, but as with any meditation book, this is best taught by an experienced meditator.