Ana Estevez claims the department is partially responsible for the death of Aramazd "Piqui" Andressian Jr., who was smothered by his father and then buried in Santa Barbara County.
Estevez's estranged husband received a jail sentence of 25 years to life for the killing.
The lawsuit alleges that the Department of Children and Family Services knew that the boy was being abused, but still gave partial custody to the father.
Estevez is suing for negligence, wrongful death and violation of state law.