By Rob Virtue
In a shocking insight into the lives of youngsters who are taken from their families to be indoctrinated into the jihadists' extreme ideology, escapees have told of how they were dressed like militants and given guns.
Taha, a 13-year-old Yazidi boy who managed to escape a ISIS training camp, described how jihadis "taught us the best way to cut a head off and where we should aim on the neck".
ISIS militants arrived in his home town of Kocho in Iraq last August. Men were shot dead while women and girls were taken as sex slaves.
Boys were taken to the ISIS stronghold of Tel Afar, northwestern Iraq, where they were trained as fighters.
Taha said children were shown videos of beheadings and made to act out decapitations on each other.
He managed to escape in April when, after being allowed out to visit his family, they all slipped out to a Kurdish-controlled region.
"If we couldn't shoot they would get angry and beat us. They told us we were ISIS fighters and had to learn."
But there are still currently around 300 boys of Yazidi ethnicity - an ancient religion in the Middle East - in the hands of extremists, according to the charity, the CNS Foundation.
Another boy held captive, 11-year-old Hassan, who was kept in a similar school in Tel Afar, said children were allowed to go swimming and play football.
However, they are also beaten with belts and given strict religious lessons on the Koran.
A third boy, Hussein said he was given a knife and a gun in Tel Afar and told to "kill the infidels".
He was then taken to a training camp in ISIS's self-declared capital of Raqqa, Syria, where he was given shooting practice.
He said: "If we couldn't shoot they would get angry and beat us. They told us we were ISIS fighters and had to learn."
That came from more Yazidi children who escaped terror training camps in Syria, who also told how they were forced to watch decapitation videos.
ISIS have put the children's training into practise and recently released a shocking video of a boy as young as eight cutting the head off of a Syrian soldier.
The latest insight comes as the British Government said 834 children in the UK are vulnerable to radicalisation.