Videotape of the dramatic rescue shot by a bystander shows two men using metal pipes to break the windshield, freeing the woman and her 2-year-old daughter.
The woman's 4-year-old son was still trapped inside. Two off-duty firefighters tried pulling the boy from the burning vehicle. One of the firefighters used a knife to cut the boy's seatbelt.
Police and fire officials say the boy suffered burns to 30 percent of his body and is hospitalized at Children's Hospital of Wisconsin. The firefighters were treated for burns to their hands and arms.