The couple with their ailing son were returning on foot to their village when a speeding truck ran over them, Sidheshwar police station in-charge Manish Kumar said.
While the 25 year-old woman Vidha Devi and her under a year-old son Shriyanshu Kumar died on the spot, her husband Ravi Kumar sustained injuries, he said.
The truck driver escaped from the spot after abandoning his vehicle, Kumar said.
The injured man has been admitted to Madhepura Sadar hospital for treatment, the police officer said.