A federal judge in Tennessee blocked the execution of a condemned child killer yesterday, hours before he was to become the first person to be put to death in Tennessee in 40 years. Robert Glen Coe was to die by lethal injection at 1 a.m. this morning for the 1979 kidnapping, rape and murder of eight-year-old Cary Ann Medlin. U.S. District Court Judge Aleta Trauger blocked the execution to consider a petition by Coe’s attorneys contending he is not mentally competent for execution. Coe’s attorneys say he is insane and executing him would violate the Eighth Amendment’s ban on cruel and unusual punishment. The Tennessee Supreme Court has found him sane for execution.