A police sergeant from Ocean County was charged with endangering the welfare of a child for allegedly leaving a handgun “unsecured and accessible to juveniles,” authorities said Thursday. Sgt. Andrew Wahlberg, of the Beach Haven Police Department, was issued a summons Wednesday stemming from the May 18 incident that happened in the Manahawkin-section of Stafford, according to Ocean County Prosecutor Bradley D. Billhimer and Stafford Chief of Police Thomas Dellane. An investigation found that Wahlberg “left a Beretta 9 mm handgun unsecured and accessible to juveniles” in Stafford, the prosecuto…