The blaze broke out just before 1.45pm in the kitchen area of the Miles Street property.
Fire crews from Chester Hill and Guildford arrived to find a large volume of smoke issuing from the rear of the home.
NSW Ambulance paramedics are treating the 20-year-old woman and her eight-month-old child.
FRNSW investigators are also at the scene.
Mum, bub badly burned in house fire
A MOTHER and her baby have been badly burned in a house fire at Chester Hill this afternoon.