A 4-year-old boy has died after police say he was struck, knocked to the ground and run over by an SUV that then fled in Southwest Philadelphia.

Police say the boy was struck around 6:30 p.m. Monday on South 57th Street in the city's Kingsessing section.

He was pronounced dead at Children's Hospital of Philadelphia.

A half hour later, a 19-year-old woman and her 2-year-old son were critically injured in a hit-and-run crash about 7 miles away in the city's Kensington section.

Police say the woman was holding the boy in his car seat as she crossed Mascher Street. They described the car as a white Nissan sedan.

Investigations into both crashes are continuing. Police say they're using surveillance video to identify the drivers.