An inquest set down for March will determine what lessons can be learnt from the horrific death of Queensland toddler Mason Jet Lee.
Mason was 21-months-old when he died days after being hit in the abdomen by his mother's boyfriend in June 2016.
Counsel assisting Jacoba Brasch has told a pre-inquest hearing the inquest will seek answers from child safety workers involved in the boy's life, including how the case could have been handled differently.
"And it may well be that no one put those pieces together to render the picture of a little boy who needed care and protection, without a parent willing and able to do so.
"If so, a fundamental task for this inquest is to identify whether the entities involved in protecting children can be better assisted to render the whole picture of a child in need."
O'Sullivan inflicted a blow on Mason around June 6, 2016.
His small and lifeless body was handed to paramedics in the early hours of June 11.
Lee and O'Sullivan pleaded guilty to his manslaughter, with both failing to get him medical help as he died.
There were only few recorded sightings of the little boy, and Ms Brasch said it was possible he might not have been seen by any child protection or support services in the days before his death.
"It does not even seem clear that those services tasked - indeed obliged - to protect children in need of protection even knew Mason was in the primary, if not sole, care of O'Sullivan, largely to the exclusion of his mother," Ms Brasch said.
"This was probably as a means of O'Sullivan exerting controlling and coercive power over her, so she would not leave."
Ms Brasch said it was clear that Lee did not tell services Mason was with O'Sullivan.
"It was a neighbour who told services that Mason was 'like a hostage' of O'Sullivan's in a phone call at about 4.30pm on Friday, 10 June 2016," she said.
In the year before Mason's death, Lee and her five children were homeless and living in the garage of someone she knew.
In April 2015, O'Sullivan presented to a hospital with police "with suicidal and homicidal ideations, threatening to 'skin' his estranged wife (not Lee), 'kill' his children and hang himself," Ms Brasch told the hearing.
Both Lee and O'Sullivan were themselves known to child protection services as young people, and as adults. Drugs and alcohol abuse were a part of both of their lives.