A council was criticised for the way it investigated complaints that a young boy was left alone with a convicted child abuser, it was reported today.
The Public Services Ombudsman for Wales said Pembrokeshire County Council was "tardy" in instigating child protection procedures.
The case, involving the child identified as A, took place five years ago.
The child's father, who had stopped living at the family home, had raised concerns to the council about the care that A was receiving there.
He then complained about council delays in investigating his concerns.
A referral from the NSPCC that the boy was being left unsupervised with a family member convicted of child abuse was not properly investigated, the ombudsman found.
The report, seen by the BBC, said: "Although social workers were aware of A's mother's family history, early assessments did not address the need to ensure that A's mother acknowledged the risks and kept her children safe.
"A referral that A was being left alone with a second close family member, convicted of child abuse, was not properly investigated."
The ombudsman said the council had been "tardy in initiating child protection procedures despite referrals from the emergency services in December 2005.
"This pointed to an 18-month delay before A was registered as a child at risk, and measures should have been taken sooner to safeguard A and to promote his welfare."
The council said it accepted the findings. A statement read: "During 2005, the social services child protection team was enhanced by the appointment of two duty managers and the following year by the appointment of a new team manager.
"While there were identified weaknesses in elements of the handling of this case, especially in recording decisions, there was no suggestion that the child suffered as a result of his relationship with two relatives who were schedule one offenders, and we are pleased that he is now responding well in his new circumstances.
"The issue being dealt with in this case took place almost five years ago and since that time successive inspections by the Social Services Inspectorate, Wales have recognised the continuing improvements in the service and the importance placed in the service by the county council."