It happened around 6:05 p.m. Tuesday at Bridge Street and Summerdale Avenue in the city's Summerdale neighborhood.
Police say a 26-year-old woman and her 4-year-old son were in the crosswalk when they were hit by a 2002 Toyota Highlander that was making a left turn onto Bridge Street.
The woman was hospitalized in stable condition. Police say the boy remains in critical condition at Children's Hospital of Philadelphia.
Authorities say the Toyota driver remained on the scene.
No arrests have been made.
Anyone who witnessed the crash is asked to call police at 215-686-TIPS.