In video evidence, the 11-year-old told the Old Bailey that his teacher Umar Ahmed Haque, 25, held regular grooming sessions at the Ripple Road Mosque.
The teacher is alleged to have shown a group of boys videos of violence between Islamic State and Britain.
“He showed us Islamic State are good,” The Times reported the boy as saying.
“He said that when we grow up, we have to fight with the Islamic State and help them stop these bombings. So you had to be together with the Islamic State.
“I was thinking we should fight against them if they are bombing us and destroying Britain and that’s when he showed us the good side of them.”
The boy said the harrowing videos featured footage of bloody fighting, with “babies getting killed, babies in hospital”.
He added: “I felt kind of like sad and determined, no offence, but to kill all the people that were . . . the Queen sometimes. But at the time I didn’t know what was happening.”
He said that he now regrets thinking Islamic State were good.
Another boy said that they were made to promise that they wouldn’t tell their parents what was happening at the mosque.
Haque is accused of researching and planning a terrorist attack, with targets said to have included Heathrow and Big Ben. He denies preparing acts of terrorism between March 25 and May 18 last year.