Cameron Rose, now 18, battered epileptic Rhys Lawrie, three, on two earlier occasions before the fatal attack in January of last year.
Callously, Rhys' mum Sadie Henry, 28, repeatedly lied about the assaults to hide her affair with schoolboy Rose, who was 10 years her junior.
Rose, who was 16 at the time, swung helpless Rhys by his leg into a hard object causing massive head injuries.
Ms Henry, a former army medic, returned to her home in Erith, Kent, to find her son's lifeless body in his blood-spattered bedroom.
He was covered in bruises and his right leg was broken. His face was so swollen paramedics could not open his eye to shine a light into the pupil. Rose claimed the injuries were caused as a result of Rhys' epileptic seizures.
As reported on Parentdish last month, Rose, of Eltham, south London, was cleared of murder but found guilty of Rhys' manslaughter and assault causing actual bodily harm in January last year. He was ordered to be detained for five years.
Rhys' grandfather, Trevor Lawrie, launched an unsuccessful legal challenge against the Crown Prosecution Service after they decided not to charge Ms Henry with murder.
She said she had only accepted the teenager's explanations because he seemed so 'calm and collected'. Rose was attending a school for emotionally disturbed youngsters when he began an affair with Ms Henry.
They'd met at a bonfire party hosted by his brother and sister-in-law, Gary and Louisa Norris, and the pair struck up a sexual relationship within a fortnight.
She said: "I was fighting against the fact he was 16 years old, I did discuss it and everyone seemed to say to me, 'Age is just a number'. If anything, they encouraged it'."
Rose then spent every weekend with her at her two-bedroom first floor flat in Columbus Square, Erith, Kent, while telling his mother he was staying with the Norris family.
He hit the boy when he was left to look after him as Ms Henry went on the school run with another son.
Four days later, Rhys collapsed after being swung by his leg and his head knocked against a wall. Miss Henry lied to paramedics and police to try to cover up her affair.
The Recorder of London, Judge Peter Beaumont, told Rose: "Rhys died a dreadful, violent death at your hands and that must not be forgotten."
The trial had been told by Miss Henry that she met Rose at other people's houses and even had sex with him in a park after Rhys died.
She was later cautioned for trying to cover up the relationship. She agreed she had been told not to have any contact with Rose, but had carried on seeing him.
She said: "I felt guilty because my three-year-old son died and the person I cared about saw him die. I thought he died from natural causes. I thought I should have been there.
"I was attracted to him. I didn't see the consequences. I didn't know I was hurting my children in the process. I was having fun for the first time. I did not see him as a toyboy.
"Now that I look back, I do see myself as irresponsible but it was not as sordid as it was made out to be."