The helicopter was headed for Children's Memorial Hospital in Chicago from Valley West Hospital in Sandwich, about 50 miles west, when it went down minutes before midnight, said Aurora police spokesman Sgt. Robb Wallers.
Wallers said the helicopter may have clipped a radio tower's wire before the crash.
Children's Memorial Hospital spokeswoman Julie Pesch initially said the child was being transported to the hospital because of epileptic seizures. She later said she could not reveal the reason for the transport because of privacy rules.
Pesch said the child was en route to the Chicago hospital after a closer hospital indicated there was no room for her there.
Wallers said the helicopter belonged to Air Angels Inc., an emergency medical transport service based at Clow Airport in suburban Bolingbrook. Telephone calls to the Air Angels offices were not answered early Thursday.