Councillors and officials said they were making no excuses and the apology was unreserved for failures in the care system
A council has apologised unreservedly to the family of young children who were sexually abused by a teenager placed with them by social services.
Vale of Glamorgan council said "there were no excuses and it should never have happened".
Three workers have been suspended while more disciplinary action will follow and others will receive extra training.
The 19-year-old admitted rape and sexual abuse at Cardiff Crown Court while on a placement scheme for adults.
By Wyre Davies, BBC Wales correspondent
The Vale of Glamorgan couldn't have been more contrite or apologetic in this report - then again, the mistake they made in placing a 19-year-old with a young, trusting family could hardly have been more calamitous.
Social services staff knew full well about the young man's previous history of disturbing sexual behaviour with children. Yet, out of what could be seen as some bizarre and misguided over-emphasis on his rights and concerns, they kept that information to themselves. A family who thought they were doing their civic duty by taking in a vulnerable young adult to act as a 'big brother' to their own children were instead welcoming someone into their home who would go on to inflict irreparable damage.
For the rape and serious sexual abuse of a little girl and boy, the 19 year old is now serving an indeterminate prison sentence. The council has suspended and disciplined at least three members of staff, although controversially not its most senior managers.
The report, supervised by the NSPCC, has been quick, thorough and is full of recommendations. The apologies offered by senior staff are genuine - the sense of shame throughout the Vale of Glamorgan council is palpable.
As, with the help of trained professionals, a family tries to get over such a traumatic episode this is, yet again, a tragedy that was totally avoidable.
The teenager was placed with the couple who were not told about his past sexual history involving young people, even though the council knew about them.
He admitted raping the couple's toddler and assaulting their daughter and was given an indeterminate sentence of imprisonment.
An inquiry published by the council found the overall practice of its leaving care team was poor and at one stage it failed to carry out a risk assessment even though it had received a written instruction from a senior manager to do so.
The council's director of social services Philip Evans said: "There are no excuses. This should never have happened.
"The criticisms in the report are fully justified not only by the evidence of failings but particularly because of the harm done.
"Some of our staff did not meet their individual and collective responsibilities for taking action to protect children. This was a tragic event.
"The investigation found that while the young man was looked after by the council as a child, the risks he posed were recognised, shared and managed correctly by social services.
"However, when he left care at 18 his full history was not transferred effectively, and decisions were made without crucial details."
Mr Evans added: "We have let down badly two children who have suffered harm."
The council inquiry began in January.
The report makes 12 recommendations for improvement and a detailed action plan.
Review of referral system for adult placement and a "robust system" put in place for decision makers
Independent scrutinty of all other placements should be completed urgently
Further training in risk management for all social services staff
Completion of chronologies in cases should be a priority task
Processes for transferring cases between social workers and teams should be reviewed
Staff should get further guidance on use of hazard alert system on database
Social services directorate to reinforce attendance for staff on child and adult protection training; a system to provide training updates; remind staff of responsibility to report instances of child abuse
Urgent review of arrangements for moving young people from children's to adult services
Welsh Assembly Government to be asked whether safeguards and guidance need to be extended to children where adult placements are made within families with children
Immediate steps to meet shortfall in management of leaving care team - and an independent review of how it works
A system for reporting regularly compliance with supervision policy
A clear system for managing all referral under all Wales child protection procedures should be put in place urgently
Source: Vale of Glamorgan Council inquiry report
The council said there would be changes to the structure of its frontline services in children and adult social services and that all the management arrangements for the "leaving care team" had been changed.
It also said an action plan over the issue was to be implemented.
The council's chief executive, John Maitland Evans, added: "This is a tragic situation, and I join the leader and all others within the council, in offering a full and unreserved apology to the parents and children affected. I'm very, very sorry.
"The failings of a small number of staff make grim reading - failure to access and share information, poor risk assessment and management, bad professional judgement, failure to follow the all-Wales child protection procedures.
"All serious shortcomings that contributed to the tragic outcomes for the children and their parents. This sort of practice is unacceptable and must never be repeated.
"I am upset that this has happened to the family. I am angry that we have made mistakes and caused this tragic situation, and I am determined that we ensure that such events are never repeated."
Further reports are due on the case from the Welsh Assembly Government.
Gwenda Thomas, Wales' deputy social services minister, said in a statement she would be giving the council report "very serious consideration".
"It is clear that mistakes have been made and I have asked the chief inspector of the Care and Social Services Inspectorate Wales (CSSIW) to advise me about how the matters raised by this report are taken further," she said.
"Assembly members will also want to know that the chief inspector is writing today to the director of social services in the Vale, requiring a response by 29th May.
"I am determined that lessons are learnt from these tragic events and that all necessary action is taken to help prevent such circumstances happening again."
She said a national review was already under way which would "provide the basis for a national overview report on safeguarding children in Wales" and would be published in the autumn.
It is known that allegations about the man's sexual behaviour were first made in 2004, when it was claimed he acted in a sexually inappropriate way to another boy.
In 2005, he is said to have sexually touched a boy while they were both living at a hostel in England.
Last year, he allegedly sexually assaulted a 16-year-old while they were staying in the same hostel.
The man, who cannot be named to protect the identity of the family's children, was ordered to serve a minimum of six years by a judge at Cardiff Crown Court.
The family said it would not comment on the report but is considering legal action.
An NSPCC spokesman said it was "deeply saddened by this tragic case" and the impact on the lives of two young children and their parents.
"On the basis of the information that has been put before us, we are satisfied that the inquiry process has been rigorous, that the inquiry report acknowledges what went wrong in this case and that the action plan seeks to ensure that this does not happen again."
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