NSW mum and baby threatened with knife

A terrified mother has been robbed by a man with a knife as she strapped her child into her car in Sydney's west.

The woman had just put her 18-month-old child in the car at a Fairfield carpark, when the armed man wearing a hoodie jumped into the vehicle and demanded cash at about 5pm on Tuesday.

The mother handed over her money and other items and the man fled on foot.

Both mother and child were unharmed, and police are investigating the incident.