The vehicle was taken after the children's mother left the car in Kempe Road, Enfield, on 2 July.
A girl, 12, managed to get out of the vehicle as the suspect drove off. The baby was found later on a doorstep in Cheshunt.
The car was later found abandoned nearby. A 19-year-old has been arrested on suspicion of kidnap and theft.
A man has been arrested on suspicion of kidnapping after a car was stolen with two children inside.