The incident occurred last evening when 9-year-old Madan fell into the tank in Kitasar Bhatiyan village. On seeing this, his cousin, Mahendra (10), jumped into the tank to save him but they both drowned, a police official said.
Mahendra's mother Kishnadevi too jumped into the tank to save the children but she too drowned, the official said.
The bodies were fished out an handed over to the family members after post-mortem, police said.