Jaxon Juarez died in April after authorities say he was abused by his 17-year-old cousin.
RELATED: Baby Jaxon: San Jose teen charged with sexually assaulting, killing his 2-year-old cousin
Jaxon was sent to live with the mother of his cousin in February as an emergency placement. Investigators previously claimed that the woman should have been barred from being a foster caregiver because of a 2014 felony conviction.
This new claim by Jaxon's family accuses the county and state of being negligent and reckless in failing to protect the toddler.
RELATED: Baby Jaxon's death renews scrutiny of Santa Clara Co. welfare system after he was killed by cousin
They have 45 days to respond before the family can proceed with a lawsuit.
Last week, Santa Clara County was moving to fire four social workers in connection with Baby Jaxon's case.