The Lion, the Witch, and the Wardrobe
Peter, Susan, Edmund, and Lucy
Although you could consider Lucy the main focus of the story, all four Pevensie children really share the stage as protagonists in this book. All four of them are needed in order to fulfill the prophecy about Cair Paravel, and for much of the book they are all together, or at least in groups or pairs, having the same experiences. Each has a role to play, and all of them work together as a unit.