Kathie LaFond was arrested for driving on a suspended license.
After her arrest, a Chicago Heights police officer gave the car keys to her boyfriend who then slammed into a tree.
Police say the boyfriend's blood alcohol level was three times the legal limit.
Kathy LaFond says she went to pick up her boyfriend, Cecil Conner, that night at a party because he had been drinking. She brought her 5-year-old son with her because she planned to spend the night at Conner's home after that.
A Chicago Heights officer pulled her over, however, and placed her under arrest for driving without a valid license.
After that, the officer took her to the station, and Conner drove the car away. He crashed into a tree in nearby Steger about 40 minutes later, killing the 5-year-old, Michael Langford.
LaFond and her attorney, Mark Horwitz, have filed suit against the Chicago Heights Police and Conner.
She says she told the officer that Conner had been drinking, and that the officer assured her he would make sure her son was OK.
"I told them that was my son was in the back seat and that I wanted to take my baby home, and I told him that Cecil was drinking," said LaFond.
"If the police officer is going to take the mother, the custodial parent out of the situation it's their responsibility, the Chicago Heights police, it's their duty to get that child to a place of safety," said Horwitz.
Conner is charged with aggravated DUI. Several hours after the crash, he tested at about three times over the legal limit.
Chicago Heights Police said in a statement: "Ms. LaFond gave no indication that Connor was impaired or that he had been previously drinking. In this tragic case there existed no visible or other evidence of such impairment to Connor to justify police in precluding his use of his own vehicle. Police verified the validity of Connor's driver's license and insurance and advised Connor that he could meet them at the police station, three blocks away, to pick up Ms. LaFond."
The boy's death has devastated LaFond.
"He was a joy. He was everything anybody would want. And he loved me. A lot. I was his everything," said LaFond.
LaFond's attorney says worked two jobs six days a week to support her child.
The Chicago Heights police officer's car was apparently not equipped with a camera or microphone.