Three-year-old Noah Clare was abducted along with 16-year-old Amber Clare, who was also reunited with her parents.
Investigators say an Orange County mother recognized the pair Thursday after seeing an Amber Alert on social media and then called it in.
"I drove about another half-mile, and as we pulled up to the school where I was supposed to be volunteering this morning I said to my son, 'I have to go back. I think it might be them," said Julia Bonin, who found the missing children, said at a Thursday news conference.
The boy's father, Jake Clare, is facing kidnapping charges and is expected to be extradited to Tennessee.
Earlier this week, investigators asked for the public's help in finding the victims.
The California Highway Patrol Tuesday issued an endangered missing advisory for Noah and his cousin Amber, after the suspect vehicle was found in San Clemente.
Noah and Amber were taken by Jacob Clare in Gallatin, Tennessee Nov. 6, according to the CHP.
The suspect vehicle was also spotted in Arizona. Tennessee officials issued an Endangered Child Alert for Noah on Nov. 9 and elevated it to an Amber Alert on Tuesday.