The boy put his head on the desk in front of him and cried as Mr Kelson began to outline the brothers' violent upbringing.
The judge, Mr Justice Keith, has heard how the brothers threatened to kill their victims, then aged nine and 11, stamped on them and attacked them with broken glass, bricks and sticks.
One victim was forced to strip naked and perform a sex act and a metal ring was used to strangle one boy.
The brothers have admitted causing grievous bodily harm with intent.
They have also each pleaded guilty to robbing one of the boys of a mobile phone and the other of cash and have admitted two counts of intentionally causing a child to engage in sexual activity.
The boys' mother, who has seven sons including the two defendants, described to a consultant psychiatrist being subjected to serious violence by their father.
Mr Kelson described one incident when her partner said he would "take a knife to her and slice her face to bits".
He added: "These threats were made in front of the children."
The barrister said his client's upbringing could be summarised by a reference in one report to his "toxic home life".
Mr Kelson said his client had been shown horror films at his home when he was as young as 10.
Police video shows the route the boys took before and after the horrific attack
He said the films were "extremely violent" and "gruesome movies in the extreme".
The barrister said the boy also had access to his father's pornographic DVDs.
Mr Kelson said his client would also drink cider, had 10 cigarettes a day and smoked cannabis grown on his father's allotment from the age of nine.
A BBC investigation has already revealed the two attackers were well-known troublemakers and that social services were heavily involved in their lives.
A Safeguarding Children Board report, seen by BBC Newsnight, found the attack could have been prevented.
Mr Kelson said he did not want to blame the state for the older brother's behaviour but said he had been "let down repeatedly".
He added: "Children, young children, should be protected from themselves and the failure here is that nobody ever seems to have protected (the older brother) from himself and (the victims) have all suffered so terribly."
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