ISIS militants in Syria forced children as young as 14 to watch videos of beheadings and beat them with cables during six months of captivity.
The Sunni Muslim militants abducted a group of children on May 29 as they returned to the Syrian town of Kobani after taking school exams.
It freed the final 25 hostages on October 29.
ISIS has captured swathes of Iraq and Syria, declaring an Islamic caliphate that erases borders between the two.
Its fighters have killed or driven away Shia Muslims, Christians and other communities who do not share their ultraradical brand of Sunni Islam.
Kobani has been besieged by ISIS for over a month.
The abuse of more than 150 children, some held as long as six months, amounted to war crimes citing testimony from interviews with four boys among the group.
The children described being forced to pray five times a day and undergoing intense religious instruction, as well as being forced to watch videos of ISIS in combat and beheading captives,the group said.
Those who didn't conform to the programme were beaten. They beat us with a green hose or a thick cable with wire running through it, it quoted one boy as saying.
The ISIS jihadist group has released at least 93 Syrian Kurds from the flashpoint town of Kobani who were kidnapped in February.
They were among more than 160 Kurds abducted as they travelled east through Syria en route for Iraqi Kurdistan