As part of a broader deterioration in child protection services at City Council, complex cases have been assigned to "children in need practitioners", states the report of a focused visit in July, published this month.
Inspectors highlight high staff turnover and poor management oversight as key drivers of poor practice.
This has led to delays, with examples of children remaining on protection plans for years, leading to increased risk.
The report acknowledges that senior managers have begun to put in place policies to assess the impact of the decline and turn the service around.
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It states: "A practice of allocating children in need cases to unqualified staff (children in need practitioners) has meant that these staff have been asked to work with, and take responsibility for, complex cases and, sometimes, inappropriate levels of risk.
"This has contributed to drift and delay in some cases.
"New senior managers have recognised that this practice is unacceptable and have already taken steps to begin to re-allocate this work to social workers.
"Newly qualified social work staff have also been expected to carry too much responsibility too early on in their development, including being given sole responsibility for child protection cases.
"The authority is seeking to stabilise the current high rate of turnover of staff through active recruitment and revised support for newly qualified staff, including the types of cases they hold."
The criticisms come two-and-a-half years after the city's children's services were rated "good" over all following a full inspection in late 2016, but when its arrangements for children in need of help and protection were rated "requires improvement".
The service was also pulled up for failing to give clear outcomes-led advice to parents who have been left confused over what they are required to do.
The report highlights five areas for improvement:
- Plans being written in a way that parents can understand and include a contingency plan should progress not be made
- Supervision of staff including independent reviewing officers, to identify delay and ensure timely progress with "supportive challenge"
- Visits to children and families are purposeful and are recorded in a way that is relevant to the plan and includes the child's voice
- Implement an effective quality assurance framework that focuses on the experiences of children and leads to an increased understanding of, and improvement in, the quality of frontline practice
- A reduction in the number of changes of social worker experienced by some children
The critical report comes eight months after the appointment of Amanda Hatton as York Council's director for children, education and communities.
Ian Cuthbertson, executive member for children, young people and education, said: "Keeping children safe is the most important role of any local authority, which is why we welcome Ofsted's visit and recommendations.
"We had identified certain issues in our children services and, as a result, have invested over £300,000 to accelerate improvements.
"I am pleased that Ofsted's letter notes this, stating positively that the council is ‘on a trajectory of change' and has ‘political support' to put effective policies and processes in place to make further improvements.
"Among the changes already made by a new senior leadership team include establishing an improvement board, commissioning a review of all cases, restructuring teams and increasing training.
"We will continue to work closely with Ofsted, our partners, staff and the Department for Education, to ensure that our children have the best possible services."