A 7-year-old girl and 12-year-old boy were riding west in the area of the 6900-block of South Stony Island Avenue in Grand Crossing about 3:40 p.m., when a white van heading south hit them in an intersection, police said.
The 12-year-old was treated by Chicago fire crews, and taken to Comer Children's Hospital in fair condition, police said.
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CFD took the 7-year-old to Comer Children's in serious condition.
The driver remained on the scene, and was not hurt, police said.
No citations had been issued early Thursday morning.
Major Accident detectives are investigating.