Marcus Vinicius Borges Emmanuel, the first person to go on trial for the killings, was found guilty on six counts of murder, five of attempted murder, two of grievous bodily harm followed by murder, and one of causing grievous bodily harm.
Under Brazilian law Emmanuel, 29, can serve a maximum of 30 years and legal sources said he had a good chance of being granted a retrial. His lawyer said she had appealed for a retrial on his behalf.
Six children died in Rio when men opened fire indiscriminately on about 70 children playing or sleeping on the steps of the Candelaria church early in July 1993. Two others were killed later at a nearby site. The attacks, rights activists said, highlighted human rights abuses in Brazil.
Emmanuel admitted to participating in the killing at the second site, but denied he was involved in the shootings at the Candelaria. "I got out my gun and fired two shots," he said.
Prosecutors alleged he took part in both attacks, seeking revenge after street children stoned his police car the day before.
The seven-member jury reached its verdict after a 22-hour hearing that went through the night under the close watch of representatives from local and international human rights groups monitoring the trial. Jurors stayed awake with the help of frequent cafezinhos, small cups of strong black coffee drunk ritually by Brazilians.
Human rights groups have called the trial a "small breakthrough" in a country where death squad killings are rarely brought to court. "It's a small wedge in the wall of impunity," said Alison Sutton of Amnesty International.
Nearly three years after the massacre, human rights activists say the number of street children or "meninos de rua" has continued to grow. Estimates range between 750 and 2,500 out of the city's population of more than 10 million.
Human rights activists say that since 1993 there has also been a large increase in the number of minors killed in Rio.
The trials of three other men accused of taking part in the 1993 massacre were adjourned at the request of their lawyers, who asked for them to be judged separately. They are due to reappear in court on 27 May.