In the category of strong, weapon-bearing females, it's a tough choice between Katsa of Kristin Cashore's Graceling and Katniss Everdeen of The Hunger Games.
Katniss has the cool bow and arrow, but Katsa would have more of a period/fantasy feel to her garb, giving her costume the slight edge in my mind.
…and we could always throw in Robin LaFever’s amazing character Ismae from the Grave Mercy series. Because nothing says Halloween like a crossbow.
Speaking of Ms. LaFevers, her middle grade books have great costume potential with Nathaniel Fludd, Beastologist and Theodosia Throckmorton from the Theodosia series:
And speaking of awesome kidlit authors, does anybody out there have triplets? Consider the lovely author, storyqueen, and blogger Shelley Moore Thomas’s Good Knight series (you MUST read Shelley’s newest book The Seven Tales of Trinket), which features three adorable and mischievous dragons:
And speaking of dragons, with the first installment of The Hobbit movie coming out this December, why not choose a costume from the Tolkien classic. Smaug, anyone?
Or you could always go super cute and be a character from A.A. Milnes beloved Pooh Bear series or Max from Where the Wild Things Are: