Windsor and Maidenhead children's services was taken over by mutual Achieving for Children - which also runs children's services in the London boroughs of Kingston and Richmond - in August last year.
The move was taken after the quality of support for vulnerable children was found to have deteriorated since the service was rated by the inspectorate as "requires improvement" in 2015.
During a monitoring visit in November, inspectors found that the senior leadership team in the department is more stable and that staff morale in "front door" social work services had improved.
The multi-agency safeguarding hub provides an effective service, inspectors also found.
However, Ofsted is concerned about the duty and assessment social work team, which is blighted by high caseloads and workforce churn, meaning children experience changes of social worker at short notice.
The quality of casework among this team is also poor. Assessments are delayed and social workers are failing to properly describe children's experiences.
The inspection team found that staff are benefitting from a more structured working environment and more approachable management. But Ofsted warns that social work supervision is variable.
"Some social work caseloads in the duty and assessment team are too high, and some children have experienced changes of worker, often at short notice," states a letter to the council from Ofsted.
"This has impacted on the quality of some casework. The experiences of children are not always adequately described in assessments and case recording."
Ofsted does note that senior leaders have already identified these weaknesses and have made some progress in reducing caseloads.
In September 2017, Ofsted and the Care Quality Commission raised concerns about support for children and young people with special educational needs in Windsor and Maidenhead.
Windsor and Maidenhead council has been contacted for comment.
Meanwhile, Ofsted also carried out a full inspection of children's services in Calderdale in November.
Calderdale has been handed a "good" rating overall after inspectors noted a number of improvements since it was last inspected in 2015, when it was judged as "requires improvement".
"Senior managers have created a culture in which staff feel safe and valued, enabling social work to flourish," states Ofsted's report.
"Progress is considerable and sustained, leading to mostly good outcomes for the majority of children in care and care leavers."
An area of particular praise is support for care leavers, which has been rated as "outstanding", having been graded as "requires improvement" three years ago.
Inspectors found that almost all those leaving care live in safe, suitable housing and the council is working well with partners to help them find education, training or work.
Two councils rated as inadequate during their last inspection were also visited by inspectors in November.
Inspectors found improvement in areas including child protection at Tameside, which was handed Ofsted's lowest judgment in 2016.
However, they warn the council that weaknesses remain in the quality of assessments and analysis of support children need.
The pace of improvement at Barnet children's services, which was rated inadequate last year, is "purposeful and steady", according to inspectors.
They also welcomed more timely and effective support being offered. But they want to see assessments and planning have a greater focus on long-term improvement for children and families.
