Shahnaj (38) and her son Afjal (5) died while her husband Habibulla and eldest son Shibu were seriously injured, they said, adding the mother and son were sleeping on the rooftop when the mishap occurred.
The explosion was so strong that cracks developed in the walls of neighbouring house, police said.
"Prima facie it appears that explosion in the battery booster might be the cause of the mishap," Kheri Superintendent of Police Akhilesh Chaurasia told PTI.