The toddler had no shoes or jacket when a passer-by noticed him walking into the street as a car approached.
Investigators say they went door-to-door and found the people who were supposed to be watching the child.
The boy's mother told police she'd left the boy and a 4-month-old girl in the care of two others while she attended parenting classes. She is not charged but police say endangerment charges are pending against her boyfriend's father and sister.
Both children were taken into protective custody.