The mother was backing out of the family's driveway in the 400 block of Exeter Place in the coastal community of Marina at about 9:50 a.m. when she heard a scream, Sgt. Bob Nolan said.
She got out of the car and saw that she had hit her young son. When emergency responders arrived, they found the mother "traumatized" and the boy badly injured, Nolan said.
He was taken to the Marina Municipal Airport, where he was airlifted to a Bay Area trauma center, the sergeant said.
Nolan said the boy had strayed into the driveway unbeknownst to his parents.
"The family members were all busy getting ready for their day and lost track of the boy," he said.
The boy's injuries are considered life-threatening, he said.