Ahmed Aslef was just 10 years old when he was lined up with hundreds of other Yazidi children outside the Iraqi village of Kocho in front of the heavily armed jihadi fighters of Islamic State (Isis). The IS (Daesh) militants ordered the children to raise their arms and then killed those with armpit hair.

Ahmed's two young sisters were sold as slaves along with other family members in the cities of Mosul and Raqqa as IS spread across northern Iraq at the end of 2014. Since the Kocho massacre, survivors have claimed as many as 800 people were killed with boys as young as 12 among the dead. Ahmed was spared but was recruited into the IS youth wing that has become known as the Caliphate cubs.

I was a very good boy - Ahmed Aslef, former IS Caliphate cub

A year on and Ahmed, now 11, is living in a safe house in Stuttgart, Germany, along with around 70 Yazidi women and children. He travelled to Germany as part of a refugee project run specifically for women and children who have escaped from IS. His mother has remained in Iraq to await news of his father and older brothers who remain missing and could still be held by IS, but are more likely among the many hundreds of dead whose remains have yet to be identified.

"With Daesh, I didn't go to school with girls. I didn't learn maths. I went to a place with lots of other children. We learned how to use weapons. We were around 60 or 70 boys – no girls were allowed," he told IBTimes UK in an exclusive interview.

Should we cease using Islamic State, Isis or Isil and begin using 'Daesh'? IS fighters and leaders hate the word - it's an Arabic acronym of "al-Dawla al-Islamiya fi Iraq wa ash-Sham" – meaning the Islamic State of Iraq and al-Shams – but when spoken Daesh sounds similar to the Arabic words translating to "the sowers of discord" (Dahes) or "one who crushes underfoot" (Daes). IS threatened "to cut the tongue of anyone who publicly used the acronym Daesh, instead of referring to the group by its full name".

"I was a very good boy. We learned to take weapons apart and put them back together again, and how to load them. We learned how to throw grenades very far away. And we ran a lot for a long time."

Ahmed was kept in captivity with his family for nine months in various locations inside Iraq and Syria, as the family were sold multiple times. While in captivity, he said his family were confined to a single room and allowed to leave only to go to the toilet.

He said his friends at the cub camps were from many different countries, including Morocco, Afghanistan, Algeria, Tunisia, Jordan and Germany. "We were all together every day and we all wore a special uniform like the older men," he said.

IS has taken control of most of the schools within its territory and has changed the curriculum. Experts say that in terms of the way terrorist groups use children and the techniques of indoctrination, the methods of IS have been unprecedented in their scale.

"I have been studying non-state terrorism for 20 years and I have never seen such a system towards indoctrination that I have seen with Isis," said John Horgan, at the Georgia State University Global Studies Institute.

Ahmed said the first task of the young cubs was the recitation and memorisation of the Quran, followed by physical training and light weapons training, and then by specialist training. He proudly recalled how he knew how to a fire a rifle and spoke fondly of his fellow pupils.

His main teacher, he said, was "an old man and very cross all the time" and he described witnessing a number of deaths. It appears the most extreme violence he was forced to witness was in Tal Afar, in Iraq, where he spent some time in captivity.

"One man, from Tal Afar, he was a very bad man. I was in Tal Afar, and there were lots of boys from my village Kocho, but also other boys and we were altogether. There were many Shiite men there, as well as Turkmen and Daesh killed all the men in Tal Afar," he said.

It would appear Ahmed performed well during his time as a Caliphate cub although he could not have "graduated" as he remained living with his family throughout his time in Raqqa. Horgan said IS can be quite strict on which Caliphate cubs graduate and which do not.

"It is one of the greatest ironies of all the armed groups using children in Syria, IS is one of the few groups that actually does make a distinction between the ages – the younger kids are basically told you can't do this yet, you must wait your time before you can go off and become a cub," he said.

If there is a ground invasion, I think the children will be the first to be pushed to the front line - Jan Kizilhan

Ahmed and his family are among the hundreds of Yazidi women and children, kidnapped from around Sinjar by IS last summer, who have managed to escape. Smuggled and bought back by secretive networks of people working inside IS, it is estimated that almost half of the estimated 5,000 captured are now free.

There is evidence that the events that began in Kocho on 3 August 2014 constituted an attempt at genocide. Hundreds of men remain "missing" and Kurdish forces who recently took back Sinjar after more than a year of IS occupation have so far found two mass graves.

The federal government of Baden-Württemberg in Germany is alone so far in responding to the crisis faced by the women and children who have returned. In a project that has been running quietly for the past six months, the region has issued residents visas for this group and given refuge to 1,000 of the most devastated survivors of IS atrocities.

Professor Jan Kizilhan, who has designed the trauma counselling programme for those on Baden-Württemberg's special quota programme, told IBTimes UK most of the children who spent a significant amount of time in captivity attended some kind of military training.

After interviewing 1,200 former captives, he said: "The viciousness and brutality of the culture of Isis has been undermined, as they have no kind of humanity towards their victims – the women or the children."

Heavily exploited for propaganda purposes, children are also malleable victims of radical messaging, Kizilhan said, and reassuring the boys that IS cannot reach them is something his counselling programme seeks to do. "I talked to a nine-year-old boy and he told me that he could not criticise Daesh because then he would be beheaded," he said.

Ahmed's release came after his family were sold back to his wider family network through the highly secretive networks of smugglers working at great risk inside IS in the spring of 2015. He is one of the older boys who has made it to Germany, as boys have become more valuable to the war effort they are getting harder and more expensive to get out.

The Caliphate cubs who are still in IS hands are expected to play a bigger part in the terrorist group fighting in Iraq and Syria, according to Horgan, and so far more than 50 have died fighting for the group. He said: "Children occupy a number of roles in the group – weapons transporters, nurse, bodyguard, suicide bomber. If there is a ground invasion, I think the children will be the first to be pushed to the front line."

*Ahmed Aslef's name has been changed to protect his identity.