Police say the teen abandoned the car and the 14-month-old child was unharmed.
Police say in a news release that the toddler's mother left the car running at a BP gas station Friday night in Milwaukee. Her baby was strapped into a child seat inside when the 16-year-old got in and drove away.
A witness followed the teen until he abandoned the vehicle and fled on foot. He was arrested a short time later.
Police are investigating whether he may be responsible for other auto thefts in the area.