The woman had parked her car in Tilehurst Lane, Binfield, when she unwittingly left the keys inside.
Firefighters were called at around 1.30pm, and had to smash a window to gain entry.
Watch manager for Bracknell firestation, Micheal Fulwell, said: “The little boy was fine he was just getting a little bewildered and upset.
“The back doors were childlocked which made it even more difficult, with these newer cars there’s really not many other ways to get into cars. We had to sent a fireman in through the window to grab the keys.”
He added both mother and child were fine, and the crew left the scene after around 10 minutes.