The decision to drive immediate and long-term improvements follows an "inadequate" rating from Ofsted this week, after inspectors found "serious shortcomings" in senior leadership.
Social workers are not supported to practice safely in the face of a "significant" increase in referrals, according to a report of the July inspection.
"Many vulnerable children who have experienced long-term neglect, and those at risk of exploitation and who go missing from home or care, live in situations of actual harm or are at risk of harm for too long," it states.
Inspectors refer to "highly committed and skilled social workers and frontline managers" who are working "extremely hard under very difficult circumstances" - giving up their evenings and weekends to see vulnerable children and complete reports.
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The report states: "This is not sustainable.
"Action by leaders has not been successful in creating an environment in which good social work practice can flourish.
"Senior leaders do not have an accurate view of the impact of high workloads on their staff."
The report refers to an overall social worker vacancy rate of 25 per cent - which is 35 per cent in some frontline teams.
Latest DfE workforce statistics show that the average vacancy rate in the South East as of 30 September last year was 14 per cent, and the national average was 15 per cent.
The council has made efforts to address the high rate, including a "relentless" national recruitment campaign that has brought vacancies down from 39 per cent, the report states.
It describes a range of training being available to staff, including a compulsory three-day session on "the foundations of practice" introduced in June 2018.
There are also monthly themed workshops which have included learning from audits, external reviews and complaints.
However, the report adds that Medway could have a better grasp of its impact, stating that it "does not rigorously evaluate the impact of training to inform its effectiveness or enable it to focus attention on areas of the greatest priority".
The move to draft in external support from commissioner Eleanor Brazil follows a deterioration in almost all areas since a previous inspection in 2015, when it was judged "requires improvement".
It also follows signs of improvement found in a February 2019 visit - focused on the council's response to contacts and referrals.
In addition, senior leaders and politicians have failed to evaluate and understand children's lived experiences across the wider service.
"These are serious shortcomings, as senior leaders did not know about the extent of the failures to help and protect children until this inspection," the report states.
Medway's director of children's services Ian Sutherland said the council fully accepts the findings, adding that the department has already started to make necessary changes.
"I recognise that immediate improvements must be made, including sustainably reducing high caseloads," said Sutherland.
"We have strengthened our improvement team to help support the delivery of rapid change.
"We have made a number of other changes already, including improving the process for reviewing each case to track progress and ensure it is being dealt with in a timely way."
Josie Iles, who took over the post of lead member for children's services in May, added: "We have already started to refresh the improvement action plan which sets out how we can best address the concerns and make improvements.
"We are also establishing an improvement board, which will have an external chair and a clearer multi-agency improvement approach."
Alan Jarrett, leader of Medway Council, highlighted how £2.6m was allocated to children's services in February this year, "in recognition of demographic pressures facing the service and to increase the number of our social workers to alleviate pressures on caseloads".
The overall children's social care budget is about £36m this year, up by £4.5m since 2017/18, added Jarrett.
Earlier this year, Brazil was drafted in to a similar role at Stoke-on-Trent Council after inspectors found that vulnerable children there were not being protected.