A former Children's Hospital Boston physician and best-selling author sexually abused thousands of his pediatric patients, according to a class action lawsuit a Boston attorney filed yesterday in Suffolk Superior Court. The suit names the hospital and Melvin D. Levine, MD, and alleges that he committed medical malpractice and sexual abuse and that the hospital was negligent in failing to properly supervise him during his 19 years there. The suit cites 40 former patients, all young boys at the time, who said that Levine performed unnecessary genital examinations. Boston lawyer Carmen L. Durso said his suit seeks to represent all children examined by Levine when he worked at Children's Hospital, from 1966 through 1985. Durso estimated there could be as many as 5,000 boys who were victims of Levine.