The baby, which was strapped into a car seat at the time, was recovered safe and well.
Police are looking for a man, who had been a passenger in what is believed to be a blue Peugeot, which pulled up nearby just before the theft.
The man steals the Volvo station wagon.
Policeman Carl Harris, of Cambridgeshire police, said: "The car was stolen in broad daylight in a matter of seconds by a brazen thief who got more than he bargained for.
"Fortunately, the child was unharmed but this was a deeply disturbing incident which could have had terrible repercussions."