The tri-borough arrangement between Kensington and Chelsea, Westminster and Hammersmith and Fulham councils was established in 2012 in response to tighter local government budgets.
But the arrangement formally came to an end in April after Kensington and Chelsea and Westminster served notice to Hammersmith and Fulham, which they claim was planning to leave the partnership.
Ofsted inspectors conducting a focused check on "front-door" children's services in Kensington and Chelsea and Westminster, found that both are well run and leaders have a strong commitment to improving quality.
This was the first visit by Ofsted to the two councils since they were both rated as "outstanding" following a full inspection in 2016.
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Inspectors found that councillors and senior managers in Westminster are knowledgeable about its multi-agency safeguarding hub (Mash), which deals with initial child protection referrals.
Social workers in the London borough felt well supported and concerns about the safety of children are swiftly dealt with and good support is put in place.
"Within Westminster, there is strong corporate leadership to improve outcomes for children and their families who are in need of help or protection," states Ofsted's report.
"Since the previous inspection of children's services in 2016, there has been a continued commitment to sustain and improve the quality of social work practice at the 'front door'. Senior leaders know the service well, as evidenced in their recent self-evaluation."
Similarly, in Kensington and Chelsea Ofsted praised leaders for having a good understanding of its Mash and areas for improvement.
They are also impressed by the organsation of social workers in Kensington and Chelsea, who are based in locality teams that include specialists with experience of complex cases and family therapists. These teams also have manageable caseloads, inspectors found.
"Highly experienced senior leaders and managers provide staff with a clear vision and focus to ensure that children and families receive good-quality services," states Ofsted's report.
It adds: "The locality social work model is a strength, as children and families receive consistent social work support for as long as required."
Among recommended improvements for both councils is to involve partners more in children's social care strategy discussions.
In December last year it emerged that the collapse of the tri-borough arrangements would leave Westminster facing an estimated £420,000 bill to form a partnership with Kensington and Chelsea.