Parenthood is scary, especially in supernatural fiction. The final devilish twist at the end of “Rosemary’s Baby” comes with Rosemary’s maternal acceptance of the son of Satan. The horror that drives Stephen King’s “The Shining” hinges on Jack Torrance’s desperate desire to be a good father. With his new dark fantasy novel, “The Changeling,” Victor LaValle explores some of the most frightening aspects of parenthood, whether mundane or fantastic.