The incident happened in Ali Abad district of Kunduz, which took the brunt of attacks by the Taleban insurgents on Saturday when the militants fired rockets as part of an effort to disrupt the parliamentary poll in many parts of the country.
Habibullah Mohtashim, the district chief of Ali Abad, said eight children were killed while playing with a rocket round left lying on the ground.
"Seven died on the spot and the other one passed away while being taken for treatment," Mohtashim said.
Mohtashim said the rocket may have been fired by the Taleban on Saturday and did not explode, or was placed for a planned attack which never happened.
The interior ministry said six children had been killed and that the rocket had been fired by insurgents on Sunday. The reason for the discrepancy was not clear.
Unexploded ordnance and mines, a legacy of the three decades of war triggered by the former Soviet Union's invasion of Afghanistan, have killed or maimed thousands of Afghans.
More than 650 sq km (250 sq mile) of Afghan territory are still considered active minefields, and more than 11,000 anti-personnel mines and 380,000 other explosive remnants were cleared in the first five months of this year, the UN has said.
While demining agencies routinely struggle to clear the past threats, new explosives and ordnance used by NATO forces, Afghan troops and the Taleban insurgents fighting them appear anew, and often harm civilians.
