Agencies missed several chances to protect a girl who died after being fed drugs by her mother, a serious case review has found.
Poppy Widdison, 4, collapsed in Grimsby and later died in 2013. Michala Pyke and her ex-partner John Rytting were both found guilty of child cruelty.
The review found council services did not know the extent to which "she was exposed to risk and harm" daily.
North East Lincolnshire Council said it fully accepted the review's findings.
A report, published by the North East Lincolnshire Safeguarding Children Board (LSCB), said Pyke "managed to mislead professionals with apparent ease".
During the trial, the court heard Pyke and Rytting, who was not Poppy's father, encouraged her to eat sedatives because they felt she was an inconvenience to their relationship.
A post-mortem examination could not establish a cause of death, but toxicology tests found she had ingested significant amounts of heroin and methadone for a period of between two and six months before her death, the jury heard.
After her birth, Poppy was due to be taken into the care of her paternal grandparents due to agency concerns, but "her legal status remained unchanged" and Pyke and Poppy's father retained parental responsibility.
The report said no "parenting assessments" were completed at this time, with no "multi-agency analysis" of the risks involved.
A "loose approach" was seen in terms of assessments, meaning her parents were able to "hide the negatives" from agencies.
The review found Pyke and Rytting both had a long history of substance abuse, with agencies unaware they were in a relationship and living together until Poppy's death.
The pair are due to be sentenced at Hull Crown Court on Monday.
Rob Mayall, independent chair of LSCB, said: "Significant changes have taken place across NEL [since 2013] and much of the learning and many of the recommendations cited in this review have already been implemented."
A spokesman for the NSPCC charity said: "It is deeply worrying that this review has highlighted ineffective management within children's social care."