Police said at least seven more children were injured on May 25, when the faulty gas cylinder exploded on the outskirts of the city of Gujrat, some 200 kilometers southeast of the capital, Islamabad.
The incident occurred as the bus was taking the children and the teacher to a school.
The office of Prime Minister Mir Hazar Khan Khoso said he "directed the authorities concerned to provide best medical care to those injured and all possible assistance to the bereaved families."
Previous vehicle explosions in Pakistan have been blamed on substandard cylinders used to contain compressed natural gas.
The fuel is used in millions of vehicles in the country as a cheaper alternative to diesel and petrol.