The children ranged in age from 1 to 4 years old.
The security guard has been named as 50-year-old Damiao Soares dos Santos. The fire gutted the day care centre.
Military police in eastern Brazil said a security guard doused himself and children at a day care with gasoline and set the facility on fire. Santos died from his injuries at the hospital. 25 people mostly children aged between four and five were hospitalised with burn injuries.
Governor Pimentel ordered the creation of a special operation to receive the victims, both in hospitals in the region and in in Belo Horizonte.
Brazilian President Michel Temer expressed his sympathies via Twitter. I, who am a father, imagine that this must be a most painful loss. "But those who have been rescued, the children and the adults as well, we are very hopeful that all will survive", Pimentel told reporters at the scene.
The suspect reamins in a serious condition in hospital after the attack - his motive remains unclear.
"This morning, four children and a teacher burned to death when a guard at the nursery school sprayed alcohol on the victims and his own body before setting them alight", the prosecutor of Minas Gerais state, in the country's southeast, said in a statement.
He allegedly came to the center Thursday to turn in paperwork before starting the blaze. "He has been an employee since 2008 and has never had a bad record so we could never have anticipated something of this nature happening", Mayor of Janauba, Carlos Isaildon Mendes, told the media.