He sustained 101 injuries including missing teeth, bruises and deep cuts. Kristy was found drowned after allegedly being hosed down in the bath by his attacker.
The young lad was on a Christmas visit to his sister Magalie in London when she and her partner Eric Bikubi, both 28, allegedly set upon him and his siblings.
The Old Bailey heard how their ordeal began when Bikubi accused Kristy, his sister Kelly, 20, and an 11-year-old sister of doing witchcraft and “adversely influencing his three-year-old son”.
Brian Altman, prosecuting, told the Old Bailey: “Despite her own siblings’ denials that they were sorcerers, Magalie Bamu joined her boyfriend in repeating these fantastic claims and participating in the assaults.
“They beat the three of them, refusing to let them eat, drink or sleep for days while the punishments being meted out became increasingly violent, with them using many implements found in the flat as weapons of torture.
“However, it was Kristy who became the focus of Bikubi’s attention and, in a desperate attempt to prevent any further suffering, he and his two sisters were eventually to admit to being sorcerers.
“Wickedly, the defendants also recruited sibling against sibling as vehicles for their violence.
“In a staggering act of depravity and cruelty, they both forced the others to take part in the assaults upon Kristy.
“The children had no option other than to do as they were told, or risk the same violence to themselves.”
Jurors were also told Bikubi and Bamu, from Newham, East London, then placed them and their brothers Yves, 20, and a 13-year-old boy in a bath to be hosed down with cold water.
Mr Altman added: “It was only when he realised that Kristy was not moving that he stopped what he was doing and pulled him from the water. By then it was too late.”
Mr Altman went on to tell how Kristy even begged to be allowed to die before being placed in the bath.
He went on: “Kirsty was just too badly injured and exhausted to resist or to keep his head above the water. It is possible he was even unconscious.”
Kristy was found dead in the bathroom while his siblings were in the living room.
Mr Altman said: “All were standing in the living room, hysterical, terrified and soaking wet.
“None of them spoke any English. Both girls appeared to be injured.”
Several calls were made to the youngsters’ parents in Paris during the alleged four-day torture on Christmas Day 2010.
But their dad was said not to have realised what was happening. When Kelly asked her sister why she was not protecting them, she allegedly said “they deserved it and that she had no pity for them”.
Bikubi and Bamu deny murder. Bikubi admitted manslaughter but his plea was rejected by the prosecution. He also admitted the assaults on the two sisters.
Bamu, who claimed Kristy drowned himself, also denies two charges of causing actual bodily harm to her sisters.
The trial continues.