Police say just before 8:30 p.m. Saturday, the striking vehicle was headed south on 6th Street and turned left onto Washington Avenue.
At that time, 20-year-old Deone Leone and her 2-year-old son Derek Martinez were coming back home from the grocery store when the car struck them.
Authorities say the two were less than half a block from their home in the 500 block of Washington Avenue when they were hit.
A witness told Action News the car was flying down the street.
"I saw the lady and the kid about to walk the street, I saw a car just going by super fast and just ran over the lady," the witness said.
Martinez was taken to Children's Hospital of Philadelphia where he died a short time later.
Leone was taken to the Hospital of the University of Pennsylvania in critical, but stable condition. She was released on Sunday.
After the car ran into the mother and toddler, authorities say it cut through a parking lot and back out to 6th Street where it was last seen speeding off southbound.
A day after the deadly hit and run, family members and a priest gathered at the South Philadelphia home of the victims.
"It's terrible, I can't believe it," victims' cousin Jorge Abelino said.
The witness said it was clear the driver knew he or she had hit someone.
"He was like stopping and starting, I saw his brake lights, so I guess he was trying to decide should he stop or not and next thing I know, he turned around that corner," the witness said.
The car is described as a late model 4-door silver Mercedes CLS with 19-inch chrome wheels and low profile tires. Police say it likely has front end damage.
Police are examining surveillance video taken from several cameras at the busy intersection.
Residents are saddened and appalled at the coldhearted act of the driver.
"A human being can hit a child and keep on going, look at that child, and spin around and keep on going, how could you?" Kimberly Washington of South Philadelphia said.
Police are asking anyone with information on the silver Mercedes CLS and the driver to call accident investigation at 215-685-3180.