A 60-year-old child murderer on death row at San Quentin State Prison has died of an apparent heart attack after being incarcerated for almost 17 years, a lieutenant said today. Robert Jackson Thompson was taken to Marin General Hospital around 8 a.m. Saturday, Lt. Eric Messick said. He had earlier complained of chest pains, and was pronounced dead by an attending physician at the hospital, the lieutenant said. An Orange County jury convicted Thompson of sodomizing and strangling to death 12-year-old Benjamin Brenneman in a 1981 case in Anaheim. He has been at San Quentin since December 1983.