Alexandra Walker, 26, and her boyfriend Harrison Simpson, 22, were convicted of murder and child cruelty earlier this week following a trial at Teesside Crown Court. Simpson was also convicted of sexually assaulting Isabelle, while her mother was cleared of the charge.

Cleveland Police has admitted that officers failed to execute a warrant issued by Teesside Magistrates’ Court for Simpson’s arrest in March 2024, The Sunday Times reported.

The warrant was issued after Simpson failed to appear in court for breaching a non-molestation order, an injunction designed to protect domestic abuse victims.

The order was understood to have been put in place after reports of Simpson threatening and harassing a woman.

A spokesperson for the force said: “Multiple enquiries and arrest attempts were carried out to try to locate and arrest him, but these proved unsuccessful.

“We recognise the time Simpson had an outstanding arrest warrant is far from adequate. A review of this was immediately conducted upon his arrest for murder.”

Cleveland Police said a number of changes had since been made to suspect management processes, including the introduction of a comprehensive data system tracking outstanding warrants and the “stringent oversight” of a governance group led by a superintendent.

Ms Justice Norton sentenced the couple to life in jail with a minimum term of 28 years and six months for Simpson and 22 years for Walker, both minus time spent on remand.

She said Isabelle was sexually assaulted at around the same time as an unsurvivable head injury was inflicted, from which she died in hospital 24 hours later.

“Any death is a tragedy,” she told the court. “The death of a very young child at the hands of those who should have cared for, protected and loved her – as she no doubt loved them – is particularly distressing for all involved.”