The incident took place in Kathalbari in Bhagalpur, about 200 km from Patna.
"Some children, who were playing in the field, found a bomb. As they were not aware it was a bomb, they hit it with a brick that resulted in the explosion and injured seven of them," Brari police station officer-in-charge K.K. Akela said.
All the injured children were admitted to a hospital for treatment.