Mount Kisco, N.Y. • A son of the late Sen. Robert F. Kennedy was acting on instinct when he kicked a maternity- ward nurse as she tried to stop him from taking his newborn son out of the hospital, a defense lawyer said Monday. In opening statements at Douglas Kennedy's harassment and child- endangerment trial, attorney Celia Gordon said Kennedy "just wanted to take his baby out for some fresh air" on Jan. 7 at Northern Westchester Hospital. Nurses overreacted abrasively, she said, leading to a scuffle. But prosecutor Amy Puerto said Kennedy had to be stopped because he was violating hospital policy. She said he refused to obey the nurses, one of whom was injured.