Serenity died less than a month after she began living with her father full time in Oklahoma City at the recommendation of DHS workers.
The girl was placed with her father, Sean Devon Brooks, even though she was injured twice in January during overnight visits with him. DHS was involved because Serenity's mother had been accused of molesting a boy.
Brooks, who did not know he was the girl's father until she was 3, has been charged with first-degree murder.
DHS officials say child-welfare workers made mistakes in the girl's case. Four workers were put on administrative leave. One committed suicide. Another resigned. The other two are in the process of being fired.
Aja, 7, was killed in January 2010 by her stepfather, Lester Hobbs. Investigators said Hobbs killed the girl's mother in his motor home in Geronimo, left in her car with Aja, killed Aja and killed himself.
Aja was visiting her mother at the time of her death. Her father had temporary custody. DHS was criticized after her death because child-welfare workers earlier in her life had pushed for her to live with her mother and stepfather even though he was a felon and there were reports the stepfather abused her.