Nelida Jasmine Hernandez is suspected of kidnapping her 5-year-old son and 9-year-old daughter -- of whom she does not have custody -- from her mother's house on Sunday, police said.
She stopped by her mother's house around noon and asked if she could take the kids to a nearby 7-Eleven store, police said.
Two hours later, she placed the kids inside a waiting car driven by an unknown woman and they drove away, according to police. They have not been seen since.
The car may be a silver or cream-colored Ford Mustang.
Hernandez lost custody of the children in June, and since then they have been in the custody of Hernandez's mother. She does not have visitation rights, police said.
The case is under investigation, and police are asking anyone with information about the whereabouts of Hernandez and her children to call Detective Sara Alanis at (408) 779-2101 or the anonymous tip line at (408) 947-STOP (7867).