As Series 6 continues filming, some casting news has come to light. Episode Four, which was previously slated to be the Mark Gatiss story, is to star Emma Cunniffe (The Lakes) and Daniel Mays (Ashes To Ashes). This site, which also identifies a child actor in the cast (Jamie Oram, playing "a troubled young boy who has a nervous disposition"), states that the episode is currently filming which could suggest that it is, in fact, the Neil Gaiman episode. This site states it is titled, What Are Little Boys Made Of?
Thanks to Pseudo and Ruther on Gallifrey Base
ETA: This casting news refers to the Mark Gatiss episode, number nine in Series 6 which has now been named Night Terrors.