Two small children were neglected by a family blighted by alcohol, drug and sex abuse as well as domestic violence, a report has concluded.
The girl and her brother, born in 2007 and 2009, are now in foster care.
But for six years they lived with either their drug-abusing mother or troubled grandmother, Cumbria Local Safeguarding Children Board said.
It concluded there were delays removing the children and placing them in foster care in 2013.
The LSCB 29-page report also revealed that signs of possible sexual abuse "slipped off the radar".
Other lessons to be learned included professionals being familiar with age-appropriate sexual behaviour and being more aware of the impact of parental substance misuse on children's safety.
The Cumbria LSCB report said: "This case was characterised by chronic and extreme neglect within a highly dysfunctional family system.
"Professionals failed to take into account other risks facing the children that were heightened by the neglectful environment.
"All of the adults had serious emotional and or physical ill-health issues that made it practically impossible for them to prioritise and safeguard the children."
The LSCB, which oversees protecting children in the county, said it was planning a conference to spread the lessons of the serious case review.
Its report added: "Neglectful parenting itself increases very significantly the risk of other forms of abuse, for example physical and sexual abuse, since such families are likely to be characterised by a lack of boundaries, poor adult supervision and little insight into the risks posed by others in the family."