TWILIGHT CHILD by Warren Adler (Macmillan: $15.95). According to the publisher, Warren Adler's novel is an exploration of human anguish on a par with "Kramer vs. Kramer." But the chief similarity is that people go to court in a battle over a child. In Adler's story, the people are grandparents who have been denied access to their golden-haired grandson when the boy's widowed mother remarries. But Adler begins by making the young mother's decision all too understandable, given the grandfather's intolerance of her remarriage and his efforts to mold the little boy into a carbon copy of his macho daddy. Adler's strength is in keeping the plot moving along. His characters have a tendency to speechify. Their motivations are laid out like road maps, their depths examined in headlights that point only to the end of the story and leave the characters themselves almost as indefinable as the occupants of a vehicle on a dark road. Fortunately, however, the story does develop, the characters come to the point of questioning themselves and their behavior and the ending is satisfying--maybe a bit contrived but not intolerably so.