The bomb exploded when it was dug up by boys, injuring three of them critically in Kahalgaon area, police said.
The three seriously injured boys were identified as Radhe Rajak (13), Gulshan Mandal (12) and Bhojal Sav (14). They were admitted to Jawahar Lal Nehru Medical College and Hospital and the other to the sub-divisional hospital at Kahalgaon, they said.
According to preliminary investigation, the bomb might have been buried underground by criminals, sources said.