Harrowing evidence from two young boys who were in the house at the time of Liam’s death was crucial to the police investigation which led to the convictions of Nyomi Fee and Rachel Trelfa.
The youngsters were immediately separated after the incident before specially trained officers carried out a series of recorded interviews with each boy.
Police have revealed it took time and sensitivity to coax the traumatised children into describing their experiences.
Officers said as they gained the children’s trust, however, it became clear they were dealing with an unprecedented level of abuse. This led to Fee and Trelfa switching from witnesses to suspects.
During the trial, the jury watched video footage of both boys’ evidence, in which they described distressing details of the violence and humiliation they suffered at the hands of the couple.
Detective inspector Rory Hamilton, senior investigating officer, said an experienced child protection officer and social worker sought advice from a child psychologist on how the boys should be questioned.
Each child was interviewed on five separate occasions in the weeks after Liam’s death.
“A lot of planning went into the interviews of both the children,” Mr Hamilton said.
“During those it became quite clear that both, along with Liam, would appear to have been subjected to a catalogue of significant abuse over quite a long period of time.
“The significance of these interviews was that clearly it took us a wee bit of time to win the trust of both children to get them to open up, and when they did what was significant was that each was corroborating each other’s [version of] events.”
One of the children said his hands and feet would be bound with cable ties to a makeshift cage constructed out of a fireguard and bars.
The abuse was so bad one of the primary-aged boys tried to escape on several occasions.
He told interviewers after he tried to escape, Fee blocked the bedroom door with a cage where she kept “vicious” pet rats.
Mr Hamilton added: “They’d gone from being very withdrawn, very wary children to being quite comfortable disclosing exactly what has gone on.
“The fact that they were separated, it’s not plausible for [young children] to get together, make up a story of such an elaborate nature and then be expected to stick to it. I’m in absolutely no doubt about what’s happened.”
The first interview took place within two days of Liam’s death before further meetings over the following four weeks.
As police built the case against Fee and Trelfa, they were alerted to additional comments which the children made to their foster carers in the following weeks.
Other evidence – including statements from relatives, friends, childminders, web data analysis and pathology – combined to present a compelling case against the couple.
Mr Hamilton said: “[The boys] were the only ones out with Rachel and Nyomi that were in the house on the day that Liam died. Without their evidence it would have been very difficult to get this to court.
“They were used to being in an environment where everything that they did seemed to get them into trouble.
“It was winning that trust from two strangers who were adults saying you can tell us and you won’t get into any trouble. They did a fantastic job, it’s not an easy job.
“The injuries that were explained and uncovered and the level of abuse were horrific. Fortunately these cases are few and far between and I would like to give an acknowledgement to Liam’s father Joseph and the family.”
The boys, who remain in foster care, are “thriving”, according to police.
Mr Hamilton said: “They’re going to school and doing really well.”