A spokesperson for Oldham Council said: “The death of any child is a tragedy, but this is a particularly heart-wrenching and disturbing case.
“We are thankful that the perpetrators of the sickening murder and abuse of an innocent child have today been convicted for their crimes.”
They said an independent Child Safeguarding Practice Review was already under way, which would examine the handling of Preston’s safeguarding.
“Our thoughts remain with Preston’s family, loved ones, and the many people affected by this case.”
Regional adoption agency, Adoption Now, said it was a “deeply upsetting case, and our thoughts are with everyone affected by these truly awful crimes”.
It said it followed “a strict and rigorous process… in the assessment and approval of adopters”.
“An independent Child Safeguarding Practice Review is being completed.”
Both defendants will be sentenced on Thursday by Mr Justice Turner.
He thanked jurors and told them, “bearing in mind what we have called upon you to do”, they would be exempt from jury service for life.