The Nightmare Children are deformed children, a brother and a sister, who stumbled by complete accident in the PRISON of Tobias Bohr, once years ago, and never awoke from their slumber ever again. Once the two children cross paths with Tobias, they became part of his army of nightmarish creatures. In short: the Nightmare Children are nothing but mindless dolls. They have no will of their own, nor real control over their own bodies.
Unlike the rest of the enemies you encounter, the Nightmare Children have no abilities/skills that can cause more then damage (like making you confused or blind for example). All they can do is to attack. They can't protect themselves in any ways/means.