Investigators think the fire was intentionally set.
Two other children ages 6 and 12, were able to get on the roof, where they were rescued.
Investigators say they found suspicious burn patterns in Patricia Finnerty's bedroom, where the fire began.
Patricia and Garrett Fnnerty were pronounced dead early Tuesday at Northwest Community Hospital in Arlington Heights.
The fire and the subsequent deaths of those family members have stunned the community. Neighbors said that it is a very close-knit neighborhood and that the Finnerty family in particular was quite social, hosting the occasional Halloween party. Kevin Finnerty was known around town as an artist and Little League coach.
As news spread, their stunned friends went to see the burned-out house for themselves.
"I just wanted to come down here with my son, Thomas. He has special needs and he's been sent home from school today and he can't fully process any of this," said Cindy Erickson, friend.
Her son was classmates with Garrett Finnerty.
"Such loss for them," said Cmdr. Kenneth Galinski, Arlington Heights Police Department.
The fire started at 3:30 a.m. Tuesday. When police and firefighters arrived, they found two of the three Finnerty children on the roof and brought them to safety. Inside the house were Patricia and Garrett, each in their bedrooms. They could not be revived. Patricia's husband, Kevin, was downstairs with severe burns. He was on life support at Loyola Medical Center until he died.
The house became a crime scene, with investigators saying the fire may be arson after suspicious burn patterns were found at the spot where the fire may have started in Patricia's room.
The surviving children are with family.
"Obviously, they're traumatized by the whole situation," said Galinski.>> Neighbors describe the Finnertys as a happy family. Kevin was an artist and had a recent showing of his work. Patricia worked as a counselor full time at Libertyville High School. Garrett's school was staffed with grief counselors."
"The children and the parents and the teachers and principal of course become a part of a whole unit. So it's a great loss for everyone," said Dr. Sarah Jerome, Arlington Heights School Dist. 25.
Garrett was set to graduate Westgate Elementary School next week. Some of the activities that had been planned to commemorate that next week have been postponed out of respect for the family.