Breaking down in tears, the teenager told her father she is terrified she will kill like the woman whose clutches she escaped as a child.
Guilty: Joanna Dennehy confessed in brutal knife murders of three men and dumping their bodies in ditches
Since then, the 14-year-old has started a new life far away from the bad memories, while keeping the truth from her little sister.
Now, Dennehy's children will have to come to terms with the shocking reality that "Mummy" is one of Britain's most notorious female serial killers, having pleaded guilty to the slaughter of three men.
Last night Dennehy's ex-partner John Treanor, inset, revealed the devastating effect the killings have had on his children - and how he faces telling his youngest daughter the whole truth.
But he said his eldest child, who grew up seeing Dennehy descend into a life of drink and drugs, fears she is destined to follow in her mother's footsteps.
"She's seen her mum's face all over the news, the papers and internet, so she understands what has happened," said John. "She's struggling to come to terms with it all.
"The other night she was crying so I tried to comfort her. She said, 'Dad is that how I'll turn out? Will I be a killer like my mum?'
"I had to explain it's not something you inherit or that a parent can pass on to a child.
"How do you tell a seven-year-old that her mother is a serial killer?"
Dennehy's ex-partner John Treanor "She's truly terrified of turning into her mother.
She went through a lot while she was living with her mum - now she's facing an even greater challenge."
John finally split from Dennehy in 2009 after her alcohol and drug-fuelled violence became too much to bear.
The pair had two daughters together. The oldest is now 14 but the youngest is only seven and was just a baby when the couple parted.
John, 37, who lives with the girls and his new wife Vicky in a three-bed house in Glossop, Derbyshire, says: "The youngest has no idea about her mother or that she's a killer.
"She hasn't been anywhere near her since she was very little. Now I'm going to have to sit her down and try and explain the whole thing to her. I'm dreading it.
John met Dennehy in 1997 when she was just 16 and living in her parents' £315,000 home in St Albans, Hertfordshire. It was the start of a turbulent on-off 12-year relationship.
He revealed Dennehy's menacing look, complete with a green star tattoo on her left cheek, came from her need to be noticed.
"She just wanted to be marked, to stand out. She wanted the notoriety. What she has done is just mad, crazy. It's like something from a horror film."
Last week Dennehy pleaded guilty at the Old Bailey to the murders of Kevin Lee, aged 48, Lukasz Slaboszewski, 31, and John Chapman, 56.