A pair of two-year-olds also died in the blaze on 6 April, alongside the 34-year-old man and a 69-year-old woman.
“It was a heavy decision,” said Greg Minger, Woodford County’s State Attorney after the child was charged. “It’s a tragedy, but at the end of the day, it’s charging a very young person with one of the most serious crimes we have. But I just think it needs to be done at this point, for finality.”
The child has not been named and Mr Minger would not discuss the nine-year-old’s sex, or relationship to the victims.
But he said he had read multiple reports about the fire before deciding to proceed with the first-degree murder charges. The child has also been charged with two counts of arson and one count of aggravated arson.
Tim Ruestman, the Woodford County Coroner confirmed the fire was started intentionally.
Mr Minger said if child was found guilty of murder, they could be placed of probation for at least five years.
“Probation, given the age, is about the only outcome that could happen here,” he said, adding that no arrest warrant would be issued for the child, who will have a lawyer appointed to represent them.
A suspect younger than 10 cannot be detained in Illinois.
To secure a conviction, prosecutors will need to prove the nine-year-old had intent to kill in advance, which is the threshold for first-degree murder cases.
Gus Kostopoulos, a defence lawyer in Chicago, said the case would prove a major challenge for prosecutors.
“Nine-year-olds don’t know that Santa Claus doesn’t exist,” he said. “They don’t know people die and don’t come back to life. I don’t know if nine-year-olds can form intent to commit murder.”
Betsy Clark, an advocate for children within the criminal justice system, was sharply critical about the charges.
She said they were "completely out of line, given everything we have learned ... especially about the brain development of children."
The decision came days after a United Nations report recommended that children under 14 should never be prosecuted, no matter the crime.
Additional reporting by agencies