The blast, which happened Sunday at the front yard of a local house in Krong Ana District of the Central Highlands province of Dak Lak, also left two other children, aged 12 and 9, injured, the district police confirmed Monday, Cong An Nhan Dan (People's Police) newspaper reported.
The four children, who are all boys, bought the explosives online and then followed instructions on YouTube on how to make firecrackers.
The explosives suddenly exploded, causing the accident, police said.
Local people rushed to take the four children to hospitals right after hearing the blast. One died on Sunday and the other was confirmed dead on Monday.
Of the two injured children, one suffered critical injures and has been transferred to a hospital in Ho Chi Minh City.
The blast also damaged part of the front roof of the house.