The Ofsted report, published today, says overall the children's department services have significantly declined since its last inspection in 2016.
It singles out several worrying aspects, including huge workloads that swamp the department's staff and social workers.
In one of the most shocking lines, Her Majesty's Inspector Linda Steele said at the time of the visit (on October 17 and 18), a total of 267 children in the county council system remained without an allocated social worker and "without having their needs unassessed".
She said: "The majority of these children have been waiting for an assessment for more than seven days and a minority have been waiting for up to four months.
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"Many of these children have not been seen for substantial periods of time and their current circumstances and safety are unknown."
In the overview of the report, inspectors state that, when children in Northamptonshire are referred to children’s social care, "they are not consistently or effectively assessed, supported or protected".
Children who have the greatest welfare and safeguarding concerns generally receive a service from the multi-agency safeguarding hub (MASH), Ofsted found. This ensures that those families and children requiring intervention are initially responded to.
However, subsequent action is "insufficiently robust and leaves some children’s circumstances unassessed for too long and potential risks unidentified."
It adds that, against a backdrop of financial uncertainty and changes in leadership services, they visited had "significantly declined" in the past two years since the single inspection in 2016. This uncertainty had contributed to significant shortfalls in social work capacity across the service, resulting in "unmanageable caseloads and high volumes of unallocated and unassessed work".
Inspectors conceded that senior leaders are aware of the serious weaknesses and have taken remedial action. But the report adds that this "has not been effective or with sufficient urgency or rigour".
Linda Steel added: "Consequently, at the time of this focused visit there was insufficient capacity in the MASH and the first response teams to meet the needs of children and families."