Emma Tustin, 32, was unanimously convicted of murdering Arthur Labinjo-Hughes at Coventry Crown Court on Thursday.
Her partner and Arthur’s father, 29-year-old Thomas Hughes, was found guilty of manslaughter, after his son suffered an “unsurvivable brain injury” on June 16 2020.
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.