Chronicles of Narnia was a great movie, both the first and the second one, so many kids (and adult kids at heart) will be eager to dress as their favorite Narnia characters. Ebay has a great selection of Narnia Costumes for all sexes and ages.
You can find costumes for Sir Peter the knight, the White Witch Costume, Narnia Lucy, Narnia Edmund costumes, or even a Kids Prince Caspian Costume.