Another 25 people - including one adult - were injured and seriously burned in the accident near the town of Fundacion, Cesar Uruena with the Colombian Red Cross told AFP.
"The injured have second and third-degree burns, and many are still in a critical condition" in hospitals in the area and in the city of Santa Marta, Urena said.
The children aboard the bus were between the ages of one and eight and were returning home after a religious service, Fundacion Mayor Luz Stella Duran told reporters.
Department of Magdalena police told AFP that the bus exploded around noon, and that the fire was so severe they had problems determining the number of casualties.
Stella Duran said that - according to survivors - the bus driver had been handling a container with gasoline aboard the vehicle. Police initially blamed the fire on a mechanical problem.
The driver has vanished, and angry locals went to his home and threw rocks at his windows, the mayor said.
Colombian President Juan Manuel Santos traveled to Fundacion after a final re-election campaign rally on Monday in Bogota ahead of the May 25 presidential vote.