The incident took place in Mahdadpur village as 30-year-old Shabnam Devi forced her two children to consume poisonous substance before herself doing the same thing, Superintendent of Police (SP) Shekhar Kumar said.
While five year-old Akshay Kumar died on the spot, his mother perished on way to a hospital, he said, adding her two year-old daughter was battling for life at a private clinic in Bhagalpur.
The woman took the extreme step in a fit of rage following quarrel with her husband Ajay Singh, Kumar said, adding a case of unnatural death has been registered in this connection.