During his investigation of the missing children, inspector Marlot frequently sees the vision of the missing girl Alice (or "Alyc" - anagram of the "Lyca" - "The little girl lost" from the William Blake poem) singing the first few verses from the popular nursery rhyme "Oranges and Lemons"
Oranges and lemons, Say the bells of St. Clement's.
You owe me five farthings, Say the bells of St. Martin's.
When will you pay me? Say the bells of Old Bailey.
When I grow rich, Say the bells of Shoreditch.
When will that be? Say the bells of Stepney.
I do not know, Says the great bell of Bow.
You can hear it even in the first seconds of the trailer:
Then again, when Marolt interrogates Billy, a street thug who was providing children to anyone willing to pay, who laughs in the inspector's face and... starts singing "Oranges and Lemons".
I know that the unsung last part of the rhyme Here comes a candle to light you to bed, And here comes a chopper to chop off your head! is supposed to symbolising executions done at dawn, but that doesn't seem to match the theme of the movie (while there is an execution shown, it is done by hanging during broad daylight)... Of course I can be wrong here.
EDIT: It seems that this rhyme can also symbolise a child sacrifice (how - is another question) which would greatly match the theme of the movie.