Some said they worked as guards, cooks, or drivers. Others denied any personal involvement, though some said that family members belonged to Isis.
It cites the case of a 17-year-old detainee, who said he worked at a restaurant in the northern Iraqi city of Mosul that served Isis members, and believed that his name appeared on a "wanted" list because jihadists took his identification so he could be paid.
"This sweeping, punitive approach is not justice, and will create lifelong negative consequences for many of these children," warned Jo Becker, children's rights advocacy director for HRW.
She said those under 18 years old recruited by armed groups should be recognised primarily as victims who should be rehabilitated and reintegrated into society.
Iraq declared victory against Isis in December 2017 after three years of costly battles that killed tens of thousands and left Iraqi cities in ruins. The country is grappling with a massive legacy from the fight.
In a rush to deliver justice to Isis's victims, the government has sped through thousands of trials of suspects in the last year.
The Telegraph has attended hearings both in Baghdad and Mosul, witnessing torture, five-minute judgements where suspects were not always given legal representation, and even cases of mistaken identity.
This article originally appeared on the Daily Telegraph.