Homicide Squad detectives are investigating a serious auto accident that occurred in Bethpage on Monday, January 18, 2016 at 1:20 P.M.According to detectives, a 2008 Nissan Altima, operated by a male driver, 65, was travelling northbound on Stewart Ave./Arthur Ave., when he veered off the roadway onto the sidewalk and struck a tree. Buckled in a child seat, in the rear of the vehicle, was the operator’s 4-year-old grandson.Nassau County Police Ambulance and Bethpage Fire Department Ambulance transported both aided persons to local hospitals. The operator and child both sustained head trauma. The operator sustained non-life threatening injuries. The child is listed in serious condition.Detectives report no apparent criminality. The vehicle was impounded for break and safety checks. Identifications of the aided persons are not being released and the cause of the accident is under investigation at this time.