Philadelphia police were called to the scene around 11:15 p.m. Sunday at 63rd and Callowhill streets.
Police say the 27-year-old father was riding a minibike with his son; neither were wearing helmets.
Police say the bike was not registered or street legal. It had no lights.
The two were going south on 63rd when a northbound SUV struck the pair as it was turning left onto Callowhill.
Neighbor Dessa Williams heard the crash.
"I saw a vehicle at the light and somebody was screaming, 'There's a baby on the ground there's a baby on the ground,'" Williams said.
Williams was in her home at the time and said the impact was so loud she thought two cars crashed into each other.
"By the time I grabbed my bathrobe and got outside and get to the corner, I see a young child laying in the middle of the street face down and the father near the pole, somebody said his leg had been severed or came off," Williams said.
The unresponsive 6-year-old boy was taken to Children's Hospital of Philadelphia where he was pronounced dead a short time later.
Police say the 28-year-old male driver was slurring his words. They say he was driving with a suspended license. He was taken to Penn Presbyterian Medical Center for treatment.
Police say they found a bottle and can of beer in the SUV. The driver is facing DUI charges.
The father was taken to Lankenau Medical Center for injuries to his head, hip and leg. Doctors say he had to have one leg partially amputated.
Police say if he survives his injuries, he may also face charges for endangering his child's life.
Williams says the image of the boy's lifeless body will forever be seared in her memory.
"I have nieces and nephews that age," Williams said, fighting back tears. "We always have kids out here in the summertime playing and just to think that he will not be here another summer just to ride his bike is really emotional."
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