Police said at about 4:10 p.m. the ambulance was driving westbound on Galena Boulevard with lights and sirens activated when it crashed into a minivan traveling south on West Street.
"It was really loud. It was a quick one-second siren and then a bang. And it just startled all of us," said Rick Cotts, witness.
Cotts and other neighbors rushed to help. Surveillance video from nearby homes showed witnesses running to the scene.
Eleven people were transported to area hospitals for treatment, police said.
Police said the driver of the minivan was a man in his early 20s, and a 27-year-old woman was in the passenger seat. There were six children in the minivan: a set of 1-year-old twin boys, and a 4-year-old, a 6-year-old, a 12-year-old boy and a 13-year-old girl.
Officials said the woman was the children's mother.
"The youngest one, he was laying down on the floor. Paramedics were trying to help him but he wouldn't respond, so they right away called an ambulance," said Arelli Saldivar, witness.
One of the 1-year-old boys and the 12-year-old boy were taken to a Chicago hospital for serious head wounds, police said. No further details about the conditions of the other occupants was released.
"Three of them, from what I saw, were unconscious. And the older sister, she was like pretty hysterical, crying. You know, 'wake up!'" Cotts said.
The two Aurora Fire Department paramedics and the patient in the ambulance were treated for non-life threatening injuries.