In findings handed down on Thursday, Deputy State Coroner Anthony Schapel said child protection officials should have at least conducted an investigation into the treatment of Korey Lee Mitchell, five, and his six-year-old sister Amber Rose Rigney.
"Even if the children were not removed from the custody of their mother, a departmental investigation would have revealed the completely unsatisfactory nature of the manner in which the two children were being cared for," Mr Schapel said.
"This family required extremely close scrutiny and supervision.
"The mother's amphetamine usage should have been the subject of an examination and have been curtailed.
"To my mind, the police should have been asked to investigate these allegations."
Steven Graham Peet is serving a 36-year non-parole period after pleading guilty to killing both children and their mother, Adeline Yvette Wilson-Rigney, in May 2016.
At the opening of the inquest last year, counsel assisting Ahura Kalali said before the murders another child of Ms Wilson-Rigney had been removed from her care after being deemed to be in a situation of serious danger.
"So why then were not Amber and Korey removed too?" Mr Kalali said.
"Had Amber and Korey been removed from the care of their mother, their deaths would also have been prevented."
Mr Kalali outlined a long history of abuse, care and neglect notifications to authorities over the care of all three children.
There were also concerns related to the development of the children, their sporadic attendance at school and in relation to their mother's drug abuse.
The inquest heard that Families SA had difficulties with continuing to intervene in the family's case because of a lack of resources.
In his recommendations, Mr Schapel also raised the issues of resources which he said were blamed for the department's failure to comply with its statutory obligations.
"It is manifest that such a situation should never be allowed to develop and be tolerated ever again," he said.
Kids Helpline 1800 55 1800 (for people aged 5 to 25)
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