Torbay's child protection boss has apologised after inspectors condemned the service as "inadequate".
Ofsted inspectors said that in some cases children were being left at risk of "significant harm" and prospects for improvement were also inadequate.
The inspectors reviewed 50 case files of the 185 children in care in Torbay.
Carol Tozer, director of children's services, said: "We can't let these children down and I'm deeply sorry that we have been found to be doing so."
The Ofsted report said in some cases "immediate risks of harm to children and young people had not been identified".
In cases where risks of harm had been identified there were "long delays" in the investigations.
The report said: "As a result, appropriate steps had not been taken to safeguard the children leaving them at risk."
It also said children subject to protection plans "were not monitored effectively as there were long gaps in social work visits".
The report said there was a "heavy reliance" on temporary staff and case loads for some social workers were too high.
The Ofsted report came after a report commissioned by the council in 2009 also said some children were at risk.
Ms Tozer said: "These children are among the most vulnerable in Torbay.
"We've been working really, really hard for the last couple of years and before in doing a root and branch review of the way we work in children's social care and the way we work with our partners."
Procedures had changed and there had been more training, she said.
She pointed out that the Ofsted report said there was "considerable momentum" towards making changes in the Conservative-controlled council.
Ms Tozer said: "They describe our action plan as comprehensive and robust.
"But the inspectors look quite rightly at the here and now.
"The very difficult message is that despite all the improvements that have been made not all children are safeguarded to the quality that they simply must be."