Two passengers on the plane suffered minor injuries, and eight people in the two vehicles outside Midway International Airport were hurt in the incident, authorities said. Five people were in one vehicle, four in the other.
The Boeing 737 crashed through a noise wall and slid onto Central Avenue, hitting the two cars, reports Chelsea Irving of CBS station WBBM-TV. The 6-year-old boy killed was a passenger in one of those cars.
The child was dead on arrival at the hospital, spokeswoman Deborah Song said. Two adults and two other children were at the hospital, their conditions ranging from serious to good, she said.
A nursing supervisor at another hospital said an 8-year-old girl was being treated there late Thursday night.
Check out local coverage of the accident at WBBM Newsradio 780's Website.
Ninety-eight passengers and five crew members were on board Southwest Airlines Flight 1248.
Passenger Mike Abate, 35, of suburban Milwaukee said after the landing he saw a father carrying an injured child and other people being taken away in an ambulance.
"That was the toughest part. We were safe on the plane, but the toughest part was to realize that someone was under the belly of the plane," Abate said.
Midway was closed after the accident, Aviation Department spokeswoman Wendy Abrams said. The airport reopened Friday morning.
According to the Federal Aviation Administration, Southwest Airlines Flight 1248 from Baltimore to Chicago slid off the runway about 7:15 p.m. at the northwest corner of the airport, through the boundary fence and into the roadway.
The airplane's nose was crushed and a severely damaged engine was on the ground, Chicago Fire Department spokesman Larry Langford said.
Passenger Larry Vazzano, 54, of the Baltimore area said the landing seemed normal at first.
"The wheels touched down and it seemed pretty much like a normal landing," Vazzano told CBS News The Early Show co-anchor Hannah Storm. "But after five or ten seconds, you realized you weren't slowing down. I was sitting by the window, saw the snow on the runway, and became a little concerned. Then we started hitting the bumps. And I guess that's where we ran off the runway."
He said some passengers used inflatable slides to get out of the plane in the blowing snow, while others exited from stairs at the rear of the plane.
The passengers were held for questioning by investigators and first responders for about three hours, Vazzano said.