A 5-year-old boy was handcuffed and hauled off to a psych ward for misbehaving in kindergarten - but the tot's parents say NYPD school safety agents are the ones who need their heads examined. "He's 5 years old. He was scared to death," Dennis Rivera's mother, Jasmina Vasquez, told the Daily News. "You cannot imagine what it's done to him." Dennis - who suffers from speech problems, asthma and attention deficit disorder - never went back to class at Public School 81 in Queens after the traumatic incident. His mom and a school source said Dennis threw a tantrum inside...