The girl became upset after the lawyer defending the accused boys kept accusing her of lying. He said her story was "complete nonsense".
She replied: "You're talking to me like I'm a little girl. You know what? I'm going home."
She is the only person who claims she definitely saw the four accused boys in the area at the time of the attack.
'An important case'
But when the girl came back into court, she told the judge the barrister Courtenay Griffiths QC had upset her. She said: "He has already upset me and he is upsetting me again - and he keeps doing it."
The judge told her: "I appreciate that you find it unfair - but this is an important case, there is a number of defendants and they are entitled to ask you questions through their barristers."
Damilola died in a stairwell in Peckham, south London in November 2000 after being stabbed with a broken beer bottle.