The children were returning from an event at an evangelical Christian church near Fundacion, about 465 miles north of Bogota, close to the Caribbean coast.
It is not known why the blaze started, but local media said the bus may have been used for smuggling petrol.
A local police spokesman, Colonel Adan Leon, said 18 people had been found alive at the scene of the accident.
But he warned that the number of deaths could rise.
Colombian media reported that some local residents complained that the firefighters had not responded quickly enough to the tragedy.
Pictures on local media websites showed the charred, smoking frame of the burned bus.
President Juan Manuel Santos said he would visit the city soon.
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