The shooting took place during a children's party at a party venue on Saturday night, local time. The youngest victim was eight years old.
The motive for the attack is not yet clear, and police have not been able to determine who the perpetrator is.
"This may be a targeted attack. Our top priority right now is to identify the suspect," police spokeswoman Heather Brent said, according to NBC News.
The federal police FBI has been involved in the case.
Stockton has around 325,000 residents and is located east of San Francisco.
