He was taken to hospital, suffering from burns, and later died.
Military police said the man also set himself alight at the building in Janauba, Minas Gerais state.
The mayor of Janauba - population 70,000 - declared seven days of mourning. With only 2 hospitals in the small town of 80,000 people, numerous more seriously hurt were taken by police helicopter to Minas Gerais' capital Belo Horizonte, more than 500 kilometers away.
A total of about fifty people were hospitalized due to injuries, 10 remained hospitalized in life-threatening conditions, said Bruno Ataide Santos, director of the hospital.
The attack - which was perpetrated by Damião Soares dos Santos, 50, who also died - was premeditated, according to police who found gallons of fuel in his home. Police are still looking into the causes of the attack.
"I woke up early to drop him at the nursery", Jane Kelly da Silva Soares told the local O Globo newspaper.
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He was reported to have mental health issues.
The guard had worked for at least eight years at the nursery school where he was not directly in contact with the children.
"This could have been worse because the babies' room was in the hall next door".
Brazilian President Michel Temer expressed his sympathies via Twitter.
"I deeply regret this tragedy with the children in Janaúba (MG)".
As the father of a school-age child, Mr Temer said he understood "this must be an extremely painful loss" for the parents.