They said Walker prioritised her new relationship over the safety of her daughter, allowing Simpson to spend time alone with Isabelle despite suspecting he was a paedophile.
Simpson claimed he had no motive for harming Isabelle, while Walker said she did not hurt her daughter and was unaware of any abuse.
Jurors reached their verdicts after about six and a half hours of deliberation.
Simpson was unanimously found guilty of murder, cruelty and sexual assault of a child.
Walker was convicted of murder by a majority of 11 jurors to one and of cruelty unanimously.
She was found not guilty of sexually assaulting her daughter.
Ms Justice Norton said each murderer would get a life sentence, with the minimum term they would have to serve before being eligible for parole to be set by her on Thursday.
The judge thanked jurors for their work in the upsetting and distressing case.
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