Camden County prosecutors say 30-year-old David Ward of Gloucester Township punched or kicked 2½-year-old Riley Cuffee, causing a laceration to the boy's kidney, while hosting a party at his girlfriend's home on July 30 while she was out of town.
She had left the boy and his twin sister - who was not injured - in Ward's care.
The boy eventually was found around 5 p.m. the following day after a party guest who had stayed over realized he had not been seen all day.
Ward claimed the boy fell down some stairs. But an autopsy determined that he died from blunt abdominal trauma.
Murder charges were filed against Ward this week, and he pleaded not guilty during a court appearance on Friday.