Agencies took "too long" to protect at least eight children who were victims of sexual abuse and "significant neglect" in Coventry, a report found
The disclosure of abuse by one child in 2015 led to a "flawed" inquiry by West Midlands Police which closed after six weeks, the serious case review found.
Coventry Safeguarding Children Board carried out the review (SCR) after perpetrators of the abuse were jailed.
It said police and children's services had made "significant improvements".
The "repeated abuse of a number of children" was uncovered when police reopened their investigation in 2016.
For one child, the sexual abuse began at age three.
The report's authors said: "Whilst the abuse was ultimately identified and appropriate responses taken to secure their [the children's] safety and support their future well-being, this took too long, for which all the key agencies shared some level of responsibility."
A detective with "significant experience" in child protection worked closely with social care staff in the second investigation.
The children were controlled by various means, such as threats to "kill a pet", the report found.
There was not "explicit recognition" by agencies that some of the children's comments and behaviour could be seen as signs of sexual abuse.
Meanwhile, adults involved were "particularly effective at keeping professionals away", with the behaviour of one mother "so threatening" she was banned from a school.
The report said there was "little doubt" many weaknesses exposed reflected the "real difficulties" Coventry's children's services was experiencing at the time. The "overstretched" service was rated inadequate in 2014 .
Coventry Safeguarding Children Partnership made recommendations, including for additional staff training around issues such child grooming and a "clear pathway" to tackle such complex cases.
It added children's services and West Midlands Police had reviewed their actions and "put in place some significant improvements which should contribute to better services for the future".