I got the tape version for my little ones to listen to in the car. They are only 4 & 5 and they absolutely loved it. The suggested age above says 9-12, but I think that is a little high. If your younger ones have any sort of decent vocabulary, and even a passing familiarity w/ the classic children's tales (or at least their tenets), they will thoroughly enjoy this book. True, they will have some of the "dry British wit" fly over their heads, but the story is undoubtedly still for kids. Sort of like watching the Simpsons with your 12 year-old: They don't get the Kennedy impersonations or social commentary, but they still enjoy watching it.
Note that the narrator of the tape is also outstanding. He reads it like a real pro, an actor possibly. The intonations, the style, his effort at switching characters back and forth (listen to the Wolf chapter for a particularly excellent example of this skill)... absolutely first rate! Highly recommended!