Inspectors said that Tameside Council's failure to ensure sufficient workforce capacity was a "primary contributory factor" to poor practice seen in the inspection.
"This failure has led to a workforce that is not suitably experienced, high staff turnover, and some practice that does not intervene to protect and support children when needed," the inspection report states.
"As a result, there are serious and widespread failings in the help and protection that children in Tameside receive."
The authority had previously been rated "adequate" for child protection, and "good" for children in care at a previous inspection in March 2012.
Inspectors said that senior leaders' actions to address high staff turnover and increased demand for services had been ineffective due to poor analysis of need.
Meanwhile, failure to maintain appropriate arrangements for the oversight of domestic abuse notifications from July 2016 onwards meant that a large number of children were experiencing unacceptable delay before their circumstances were considered by a suitably qualified and experienced social worker.
And for some children poor application of thresholds resulted in multiple repeat contacts to children's social care, children receiving services at the wrong level of need, allegations of abuse that were not investigated properly, and children at risk of child sexual exploitation receiving services that were not informed by appropriate assessments of need.
"As a result, the risk to these children was not adequately identified, responded to or reduced, and some remained at risk of significant harm for too long before effective action was taken," the report states.
"Senior leaders' initial responses to concerns raised by inspectors in individual cases did not demonstrate an understanding of the detrimental impact on children of poor practice and thresholds for intervention that are too high."
Social workers' caseloads were deemed to be too high, meaning that social workers often did not have sufficient time to understand fully and respond to children's circumstances.
Despite the concerns, a number of areas of good practice were identified, with adoption services receiving a "good" rating.
Inspectors said that the "timely" and "effective" work of the adoption team compensates for the previous poor experiences of children through the provision of well-planned adoptive placements and extensive ongoing adoption support.
"Children in private fostering arrangements have their needs responded to well," the report states.
"Children with disabilities receive a good-quality service from a specialist multi-agency integrated team."
Stephanie Butterworth, director of children's services at Tameside Council, said: "Keeping vulnerable children safe is our number one priority, so it is reassuring that Ofsted did not find evidence of immediate harm. But our job is to do much more than that.
"The ambition of our committed and passionate workforce is not only to keep vulnerable children safe but to give them the opportunities available to all children and prepare them for a fulfilling adult life.
"To ensure we do that the voice and experience of children will be the guide for how we plan and change services to meet their needs."