How does a child endure the trauma of a volatile custody battle? What Maisie Knew, loosely based on Henry James’ 1897 novel of the same name, answers that question by putting central emphasis on the titular character, a precocious young girl caught between her two warring parents in the final days of their poisonous marriage. Sometimes, Maisie (Onata Aprile), is nothing more than a bargaining chip alternately used by her father, art dealer Beale (Steve Coogan, affecting his finely-tuned persona of self-absorption), and her mother, has-been rocker Susanna (Julianne Moore), in their legal and social skirmishes; at others, she’s a silent witness to the selfishness of grown-ups and, to a…