Victoria Police are hunting for the man after the incident in Rowville, Melbourne's southeast, on May 10.
The woman was getting out of a silver Suzuki S-Cross in the car park at Stud Park Shopping Centre around 6pm when the man approached her, police said.
He threatened her with a knife and forced her into the driver's seat of the car.
The man got into the back seat with the woman's child and told her to drive to electrical stores and demanded she buy him Apple MacBooks.
The stores included one on the South Gippsland Highway in Cranbourne and one on Frankston Dandenong Rd in Dandenong.
While the woman went into each store, the man stayed in the car with her baby.
The woman then got into the back seat and the man drove from the Dandenong store to Robert Booth Reserve, where he left the car and fled.
He was last seen after 8pm that day on the Dandenong Creek path carrying four laptops.
The woman and her child were not physically injured during the ordeal.
Police are asking anyone with information about the man, who is of Asian appearance and aged between 20 and 35, to contact Crime Stoppers.
He was wearing a grey hoodie, green fluorescent vest and black or grey pants with black running shoes.
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