Police say the 11-year-old jumped on the back of the attacker, who then turned the knife on the child.
Both victims had been sleeping in their rooms when the mother awoke to find the suspect in her bedroom.
It happened in the 2500-block of North Monitor Avenue.
Evidence of the early morning home invasion that left the mother and her son seriously injured was everywhere Friday.
"It's scary now because I'll be more aware," neighbor Theresa Bebis told ABC7 Chicago.
Witnesses say it all happened around 1 a.m. Friday as the woman and her child slept. That's when investigators say the attacker entered the family's second-floor unit and got into bed with the 40-year-old woman.
When the woman woke up and realized the man was not her husband, she began to scream. That's when authorities say the assailant-- armed with a knife-- began slashing the woman in her face, neck and body.
" She just ran, some guy trying to break into her house," said a resident who identified himself as Oscar.
Neighbors say when the son heard his mother's screams, he raced to her bedroom and tried to fend off the attacker. The attacker then turned on the boy, who was stabbed in the face, chest and hands as he tried to defend himself.
There were squad cars everywhere, ambulances," Oscar said.
The unidentified attacker escaped by running downstairs, kicking in the door of the first-floor tenants before running through their apartment and out the back door.
"I always make sure my windows are locked, and I carry my keys outside and my phone and everything, but it's very scary until they have caught this guy, " Bebis said.
Police did manage to find a knife near the home that possibly could have been used in the attack.
No one was in custody Friday night, and police said they had made no arrests.