Like all of Venezia's books, this is a great introduction to the musician. The story is in simple to understand language so young children can easily grasp what is being explained. The only caution I would give is in regards to the "comics." Considering a few incidents I've had with my preschool aged children, I would advise people to make sure to emphasize the make believe aspect of these parts when reading them to the younger children.
For older children it is a great music appreciation lesson if you add some listening afterwards. With this book on Mozart I'd recommend listening to and reading about one of his opera's since at one point these are singled out for attention.