The Government has taken control of Croydon's children's services after a damning Ofsted report revealed "widespread and serious" failures were leaving youngsters at risk.
It blamed weak management at all levels of the south London borough for failing to ensure social workers followed protocols for missing children and those at risk of sexual abuse.
Too few young people who ran away were spoken to by a social worker when they were found, inspectors reported, so the council had little data on the other risks they might be facing.
Children were also forced to wait for unacceptable periods of time for help, the report said, leaving them at risk of greater harm.
Inspectors also found many social workers were saddled with unsustainable caseloads, meaning they were unable to give enough time to each child and leading to high staff turnover.
Foster carers also complained they were not being properly supported by the local authority, and that fostering was not being properly regulated.
The report said: "When circumstances for some children do not improve, the local authority is either too slow to take action or reduces the level of support without evidence of demonstrable progress. This means that some children remain in harmful situations for too long.
"Too many children wait too long for a decision to be made as to whether they need to be looked after, or they return home without sufficient support. This has left them at risk of significant harm from neglectful parenting."
The Government has appointed Eleanor Brazil - who worked with Haringey Council following the Baby P scandal - to improve children's services in the borough.
Robert Goodwill, minister for children and families, said: "Keeping children safe is vital, and we take tough measures when councils are failing them.
"We have appointed a commissioner in Croydon to conduct a three-month review before we determine the best next steps to ensure improvements are made for vulnerable children and families."
Barbara Peacock, executive director of people at Croydon Council, said: "We accept the findings of this report and are committed to making sure that we provide better support for our children and young people. I'm sorry that our services have not been good enough.
"We identified the need for improvements last year but despite working extremely hard to make these necessary changes, they have not delivered the impact we wanted.
"The report has shown the extent of work that is needed. Much of this work is already under way but we recognise there is a lot more to do and we are working with Ofsted to create an improvement plan to drive through those changes.
"So that we and residents can feel reassured about the safety of all the young people we are involved with, we are reviewing cases and are taking immediate action where we do find issues to address."