Tustin (32) was unanimously convicted of murdering at Coventry Crown Court on Thursday, while her partner and Arthur’s father, 29-year-old Thomas Hughes, was found guilty of manslaughter.
In a call to emergency services she claims that Arthur had repeatedly banged his head after throwing himself on the floor and had headbutted her.
In police bodycam footage, she lies again about Arthur's fatal injuries just minutes after the attack, as she tries to cover up her own involvement by blaming the child.
“Basically, my six-year old stepson, he’s banged his head,” she tells the emergency operator.
“And while he’s on the floor he’s banged his head another five times. He’s lost colour..he’s got a big lump on his head.”
When asked is Arthur awake, she replies, “he’s half in and out. But he’s banged his head quite hard.”
Asked is his “breathing noisy or abnormal?” she responds, “he’s barely there, he’s barely breathing. He’s in and out. He threw himself on the floor. He’s headbutting the floor. He has headbutted me in the process.”
In the police bodycam footage she again repeats the lies and tries to put the blame for Arthur’s injuries on the boy.
“Arthur he was told to sit on the thinking step,” she tells police.
“He threw himself all over the floor, he’s banged his head off the radiator. He’s hit me, kicked me in the process of getting him back on the thinking step. He’s then gone on all fours on the floor and I told him to get up.
“I shut the door over and all I heard was a crack off the floor and he was headbutting the floor on all fours,” she adds.
“I said, ‘Arthur you’re going to hurt yourself, I’ll phone your dad, and he went ‘I don’t care’.”
Arthur died hours later, on June 16 2020, after suffering an "unsurvivable brain injury" inflicted after his head was "banged repeatedly against a hard surface".
His “wicked” stepmother and “pitiless” father have since been convicted of cruelly abusing and killing Arthur.
In court, the pair had been described by prosecutors as “utterly ruthless, unthinking and pitiless."
Speaking after the verdicts, Arthur’s maternal grandmother, Madeleine Halcrow, called them “wicked” and “evil”.
She also described the couple’s behaviour, which included Tustin poisoning the youngster by force-feeding him salt-laced meals, as “unfathomable."
“I think they are cold, calculating, systematic torturers of a defenceless little boy. They’re wicked, evil. There’s no word for them, especially your own child,” said the grandmother.
Tustin carried out the fatal assault while in sole care of Arthur at her home in Cranmore Road, Solihull, callously taking a photograph of the youngster on her mobile phone as he lay dying in the hallway, then sending the image to Hughes.
She then took 12 minutes to call 999, instead first ringing Hughes, then lying to medics that Arthur “fell and banged his head and while on the floor banged his head another five times”.
Tustin later claimed he must have thrown himself down the stairs, despite evidence he was barely strong enough to pick up his own bedding, or stand.
Hughes, of Stroud Road, Solihull, was convicted of encouraging the killing, including by sending a text message to Tustin 18 hours before the fatal assault telling her “just end him”, then later saying to his son “Watch you little c***, I’ll bury you six feet under.”
On one occasion before his death, Arthur told his father: “I am in danger with you, dad”.
Jurors also convicted Tustin of two counts of child cruelty, including salt-poisoning and withholding food and drink from Arthur.
She had already admitted two other cruelty counts; wilfully assaulting Arthur on three occasions and isolating him, including by forcing him to stand in the hallway for up to 14 hours a day as part of a draconian punishment regime.
Hughes, who had denied any wrongdoing, was also convicted of the cruelty offences which Tustin had admitted – but cleared of withholding food and drink, or of poisoning his own son with salt.
It emerged at trial that Arthur had been seen by social workers just two months before his death, after concerns were raised by his paternal grandmother Joanne Hughes, but they concluded there were “no safeguarding concerns”.
Jurors took six hours and 15 minutes to deliver verdicts, and afterwards held a minute’s silence in Arthur’s memory.
An independent review is now under way into the authorities’ contact with Arthur before his death.