The children - aged one and two - were murdered by men with a known history of drug abuse and violence.
Police, children's services, probation and health agencies were among agencies implicated in the serious case review reports.
The reports said a number of failings contributed to a lack of understanding among agencies about the level of safeguarding risks involved.
These included a preoccupation by professionals with the behaviour of the men rather than on the safety of the children.
The damning reports follow a Department for Education announcement last month that Ofsted-rated "inadequate" children's services in Northamptonshire would be turned over to a new independent children's trust.
This trust creation is part of a wider set of proposals to reorganise services, currently delivered via a two-tier system of county, borough and district councils.
In the serious case review of Child Ap, who was murdered by her mother's new partner in Kettering in April 2018, the professionals focused on "the behaviour of the adults involved in the case, which sadly was to the detriment of the wellbeing of the children," the report said.
"It is clear that there could have been better information sharing by agencies to inform a holistic assessment of the safeguarding risks presented to Child Ap and her siblings," the report's author concluded.
A report into the death of Child Ak, who was murdered by his father in Northampton in December 2017, found his case failed to reach the threshold for a Section 47 inquiry after police discovered the child left in a property where drugs were present.
It said a social worker was allocated instead to the family but up until two days before his death, no observations on his welfare had been recorded.
"[Child Ak's] safety was seriously undermined with lost opportunities to place him at the centre of any analysis of risk," the report said.
A multi-agency discussion meeting, which took the decision not to escalate the support around Child Ak, had also failed to "fully appreciate the significance of [father's] chronic history of domestic abuse and extensive history with the police for drug-related offences", it added.
The reports, published by Northamptonshire Safeguarding Children Board (NSCB), identifies a number of "local strategic level factors" which impacted on children's social care services at the time of the children's deaths.
These included a high turnover of staff, large numbers of agency staff and significant levels of sick leave.
Keith Makin, chairman of the NSCB, said: "Perhaps chief among the learning from this tragic case is how agencies need to improve information sharing within their own organisations as well as between partners."
He said the NSCB had outlined key recommendations to the council, whose children's services were last year taken over by Children First Northamptonshire.
"The report recommends a further review of training to ensure ‘think child' is front and centre when it comes to the way safeguarding professionals approach every case. There can't be a clearer message than that," said Makin.
Sally Hodges, Northamptonshire's director of children's services, apologised for the tragic deaths of both children which she said were "a matter of considerable shame" for the council.
Referring to the case of Child Ap, Hodges said: "There is simply no excuse that agencies failed to share information amongst each other which if put together earlier would have led to a far more realistic picture of the risks this child was facing.
"For our part in this failure and for the poor decisions made within social care we are truly sorry. We let this child down."
Hodges said management practices across children's services had been overhauled and improved which had led to increased timeliness of assessments and visits.
Referring to Child Ak, she said: "Such improvements mean issues which arose in this case would have been identified and escalated to ensure a more appropriate course of action."
The council has been beset by problems in recent years, including pressure to make £70m worth of savings. It was also subject to a Section 114 notice, banning it from any new spending outside of children's and adult social care.
In 2013, its children's services department was judged "inadequate" by Ofsted in 2013, and upgraded to "requires improvement" in 2016.
The department was also found to have "substantially declined" in October last year, when Ofsted returned for a focused inspection.