I suggest breaking it into separate books too if it is like Magic Treehouse. My experience has been that younger kids prefer to read many small books rather than one larger book. I suspect that this has to do with text size (which is smaller in bigger books and harder on young eyes) and weight of the book. It's comfortable to throw an MT or two into one's backpack when you are still small, less comfortable to carry around HP. I also think that younger readers like the feeling of finishing multiple books and the anticipation of the next book.
