Noah Cuatro's great-grandmother, Evagelina Cuatro, brought the case on behalf of herself and the boy's sister and two brothers, all minors. The lawsuit filed on Wednesday claims the department ignored a court order to remove him from his parents' custody, directly leading to his death.
Hathaway-Sycamores Child and Family Services, a non-profit that provides mental health services for the county, was also named in the lawsuit.
The boy's parents, Jose Maria Cuatro Jr., 28, and Ursula Elaine Juarez, 26, were indicted in January on one count each of murder and torture in Noah's death.
The indictment also charges the boy's father with one count each of assault on a child causing death and a newly added count of sexual penetration of a child under 10, with the indictment alleging that the latter crime occurred on the same day the boy was attacked.
The boy's mother is additionally charged with one count of child abuse under circumstances likely to cause death.
The inquiry began as an investigation into the accidental drowning of the boy. But in the six months after Noah's death, investigators said the child was not only murdered but tortured as well.
Noah's death occurred after multiple reports of abuse had already been made to the county Department of Children and Family Services, according to the suit.
"Instead of protecting Noah and his siblings, DCFS continued to place the children with their abusive parents, where the children continued to be abused over the course of several years,'' the suit alleges.
The DCFS previously issued a statement regarding Noah's death.
"At any given time, the Department of Children and Family Services serves more than 34,000 families and vulnerable children in Los Angeles County with an unwavering commitment to pursue child safety every day in our communities,'' the statement read.
City News Service contributed to this report.