Thomas Gorman, 39, and Carmen Medina, 31, were killed Tuesday night in the Kensington neighborhood, WCAU-TV, Philadelphia, reported.
The shooting began when Medina got into an argument with a man on the street who followed her back to her sport utility vehicle and shot her.
"She started to walk away and she was getting back into her vehicle," Capt. James Clark. "This male came upon her, opened up the driver's side door and shot her several times."
Clarke said Gorman, who was in the front seat of the SUV, put his body in the way when the gunman aimed at Medina's 11-year-old son in the back seat.
Gorman and Medina were struck several times with bullets from a large-caliber, semiautomatic handgun, investigators said. Medina's body was on the ground outside the vehicle and Gorman's was inside.
The boy, whose name was not reported, was unhurt.
No arrest had been made late Wednesday.
Medina had a history of drug arrests and convictions, The Philadelphia Inquirer reported.