A Plague of Bogles — Catherine Jinks

Jem Barbary spent most of his early life picking pockets for a wily old crook named Sarah Pickles -- until she betrayed him. Now Jem wants revenge, but first he needs a way to earn his living. He'd like to work for the bogler Alfred Bunce, who kills the child-eating monsters that lurk in the cellars and sewers of old London. But Alfred has wanted to give up bogling ever since he almost lost his last apprentice, Birdie McAdam.

As more and more children disappear under mysterious circumstances, though, Alfred, Jem, and Birdie find themselves waging an underground war. Soon they discover that there's only one thing more terrifying than facing a whole plague of bogles: facing the sinister people from Jem's past...