Midnight Robber by Nalo Hopkinson

It's Carnival time and the Caribbean-colonized planet of Toussaint is celebrating with music, dance, and pageantry. Masked "Midnight Robbers" waylay revelers with brandished weapons and spellbinding words. To young Tan-Tan, the Robber Queen is simply a favorite costume to wear at the festival--until her power-corrupted father commits an unforgiveable crime.
Suddenly, both father and daughter are thrust into the brutal world of New Half-Way Tree. Here monstrous creatures from folklore are real, and the humans are violent outcasts in the wilds. Tan-Tan must reach into the heart of myth and become the Robber Queen herself. For only the Robber Queen's legendary powers can save her life . . . and set her free.