The crash happened at about 7:45 a.m. on Lincoln Street just north of Homestead Road, police Lt. Matt Hogan said.
Hogan said a white Honda Accord driven by a 26-year-old San Jose man broadsided a white Ford Focus containing five people, causing that car to careen out of control.
The Ford drove up a curb and struck the perimeter wall of the Carmelite Monastery, knocking unconscious a 6-year-old boy who was sitting in the back seat, Hogan said.
Emergency responders found the child unresponsive, but revived him before he was transported to a hospital. He suffered "significant and traumatic injuries," Hogan said.
The boy, along with the other four occupants of the Ford, was transported to Santa Clara Valley Medical Center.
As of this morning, the boy was in serious condition, hospital spokeswoman Joy Alexiou said.
The other occupants of the Ford -- a 35-year-old Santa Clara woman who was driving the car, another woman and two young girls -- suffered moderate injuries, Hogan said.
The driver of the Honda Accord was not injured.
The cause of the collision is under investigation.