The children died Thursday when trying to open the bombs with a pair of pliers, Nguyen Van Luong, chairman of the local People's Committee in Bo Trach District, Quang Binh province said.
"The bombs look like toys," he said. The boys found them in a field when they were feeding the family's cows.
Cluster bombs dropped by the US military during the Vietnam War can contain hundreds of bomblets, according to the Mines Advisory Group (MAG). Some look like balls, while others are the size and shape of a torch battery.
The United Nations (UN) says 104,000 Vietnamese have been killed by bombs, landmines and artillery shells since the end of the war in 1975.
According to the Vietnam-based charity Renew, 10% of the 15 million tonnes of munitions dropped by the US during the conflict failed to detonate.