The incident took place at a village near Chainpur police outpost of the district when the victims, all residents of Jiradei in Siwan district, were returning from a wedding, Saurabh Kumar, SP Siwan said.
The driver of the dumper fled after the accident and search is on for him, the SP said.
The mishap took place some 25 km from the district headquarters under Siwan police station area, Kumar added.