A Weekend Herald investigation last year revealed found different state agencies had regular contact with the dysfunctional household but failed to act decisively at several crucial moments.
This included 20 notifications to CYF, including 10 for violence, but did not remove the children.
The boy, who was 13 at the time, went on to fatally stab Mr Kumar in a botched robbery of the Railside Dairy in Henderson and was convicted of manslaughter.
His 12-year-old co-accused was acquitted.
Social Welfare Minister Anne Tolley ordered a review of CYF's interaction with both families and released a summary under the Official Information Act.
Chief Social Worker Paul Nixon made a number of key findings which include:
*Neither boy was assessed properly which "significantly affected" decision making.
*Different agencies did not share information.
*No recognition of the boys' behaviour as a risk.
Both families "demonstrated skill" in navigating their way around CYF and highlighted how difficult it was to cope with the needs of children with challenging behaviours, Mr Nixon wrote.
"CYF cannot do this work alone. These cases called for clear, robust social work and inter-agency working, needing a well-planned and holistic assessment and strong integrated plan that was well implemented and regularly reviewed. It is evident that these core practice steps were not achieved.
"There were a number of distinct opportunities when a different approach may have seen the boys go down a different path."
In an interview with the Weekend Herald, Mr Nixon acknowledged the grief of the Kumar family and said CYF "clearly could have done better" with the two boys who tried to rob the dairy.
"Would that have prevented [Mr Kumar's death]? It's not clear. But if we had done a better job, it would reduce the risk."
There was poor communication within CYF but also between a number of different agencies dealing with the boys, Mr Nixon said.
"Everyone has a different piece of the jigsaw. If we don't share the information, we aren't able to put together the full picture."
In terms of the boy convicted of manslaughter, evidence given at the High Court trial where he was acquitted of murder showed there were 20 notifications to CYF.
He also attended eight schools in eight years, suffered a serious brain injury which was not rehabilitated, and expressed suicidal thoughts but was not treated by mental health specialists.
He living with his mother, who was selling synthetic cannabis at the home where they lived. In a bid to wean him off the synthetic cannabis he was addicted to, she gave him marijuana.
In sentencing the now 14-year-old killer to six years in prison, Justice Graham Lang said his childhood was "turbulent in the extreme" and helped explain the jury's verdict to acquit him of murder.
Lawyer Maria Pecotic said her client was repeatedly let down by his mother and "overlooked" by the state agencies who were supposed to provide a safety net for him.
She later successfully appealed for permanent name suppression and to cut his prison sentence to four and a-half years.
"This case is the epitome of neglect and the failure of the welfare state. And at what cost? Two lives," she said at the time.
In releasing the summary report, Social Welfare Minister Anne Tolley said the major issues are consistent with those identified by the expert panel she commissioned last year to overhaul CYF.
Youth justice will be one of the core functions of the new, as yet unnamed department, which has a much wider brief and an annual $1.3 billion budget to buy extra education, health, employment and social services for the families of all "vulnerable" children.
One in five children are known to CYF by age 17 according to the expert panel led by Dame Paula Rebstock.
Railside Dairy Killer CYF review
Planned and systemic assessment
Planned, integrated, comprehensive and well informed assessment was absent for both boys. This significantly affected the quality of subsequent decision-making, planning and intervention.
Putting a team around the child
Agencies worked largely within their own silos - collective effort and action was missing. Consequently information was not shared in a timely or purposeful way; there was no sense of shared responsibility and no collective understanding of the lives of the two boys and what was required to bring about change.
Providing an effective response to children with challenging behaviours
The boys' behaviour was not recognised as a risk of harm and caregiver capacity was not considered. Care or protection is about more than abuse and neglect - a willing caregiver unable to manage a child with challenging behaviours is unable to provide safe care.
Supervision and practice leadership
Supervision that supports critical thinking and reflection; provides both support and challenge; ensures competent and accountable practice was not consistently available or provided for the social workers and supervisors.
Source: Review by Chief Social Worker Paul Nixon
