Sinjar, Iraq (CNN) Sinjar is yielding up its secrets -- the whispers of those who prayed as they were killed. Grief for those captured and never seen again.

Mahama al-Shangali is the mayor of this town of fewer than 90,000 people in northern Iraq, in the heart of Yazidi country.

When ISIS swept through the Yazidi homeland, it was along the road into Sinjar that men, women and children who had been rounded up from the surrounding villages were driven.

Evidence of a massacre

As the mayor walks us to the other side of the heaped earth defenses that encircle the town, he tells us this was the site of an ISIS massacre.

This is where they buried the young men, the women and the boys -- children, really -- who refused to accompany ISIS. Who refused to be conscripted by the terrorist group as child soldiers.

Witnesses tell CNN that the victims in in these graves, more than 130 of them, had been selected to be taken to the nearby ISIS-controlled town of Tal Afar.

But they refused to go. So they were killed, young and old alike.

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Standing at the gravesite, I could still see tossed on the ground the cloth ties that bound their hands. The prayer beads they clutched until the end.

And I saw the empty bullet casings spit out by the guns fired by their killers.

Some Yazidis living in camps

From there we moved to a refugee camp, one of the many that now dot northern Iraq. Those who managed to flee ISIS have found refuge here.

Kurdish authorities tell CNN they have evidence that about 600 children were abducted from Sinjar and the surrounding Yazidi villages.

Around 200 have since escaped and are sheltering in camps like this one across the Kurdish region.

The Yazidis are linked to the ancient religions of this region. They believe in a single deity and a world ruled by seven angels, chief among them Melek Taus, the Peacock Angel.

ISIS considers the Yazidis to be devil-worshippers and wants to eliminate them. Killing and forced conversion of the Yazidis are espoused by ISIS as religious duties.

Setbacks prompt ISIS to seek more child soldiers

But ISIS' desire to recruit children goes beyond that. U.S. military sources tell CNN that, with the loss of the key Iraqi city of Ramadi , the militant group is feeling the pressure. So it is increasingly pulling experienced fighters to the front line and replacing them in sentry positions -- and on suicide bomb squads -- with children.

Beyond the prayer beads and the bodies, the testimony of those still living is haunting.

Photos: Fleeing ISIS: Life for Yazidi child refugees Photos: Fleeing ISIS: Life for Yazidi child refugees The Sharia Camp in Dohuk, Iraq houses 18,575 people in 4000 tents. It opened in 2014 and includes four schools -- two Arabic, two Kurdish. Many residents have escaped from their home in Sinjar after ISIS invaded and captured many Yazidis. Three-year-old Farhad Naifg is from Sinjar. He has been living in the camp for a year.

Hide Caption 1 of 9 Photos: Fleeing ISIS: Life for Yazidi child refugees Two-year-old Basman Haidar and his grandfather Haidar Kharmish, have also spent a year in the camp.

Hide Caption 2 of 9 Photos: Fleeing ISIS: Life for Yazidi child refugees 9 year-old Katrine Hamou and her two-year-old brother Zimar Hamou are from Sinjar.

Hide Caption 3 of 9 Photos: Fleeing ISIS: Life for Yazidi child refugees Three-year-old Muna Rasho is from Sinjar. She came to the camp a year ago with her three brothers and two sisters.

Hide Caption 4 of 9 Photos: Fleeing ISIS: Life for Yazidi child refugees Marwa Khalil, who is eight years old, escaped from Sinjar with her family. She suffers from a skin disease and can't go to school because other children refuse to sit next to her.

Hide Caption 5 of 9 Photos: Fleeing ISIS: Life for Yazidi child refugees Miada Murad is eight years old, from Sinjar. She has five sisters. Hide Caption 6 of 9 Photos: Fleeing ISIS: Life for Yazidi child refugees Gawre Murad, 40, and her son Aryan, four, escaped from Sinjar a year ago.

Hide Caption 7 of 9 Photos: Fleeing ISIS: Life for Yazidi child refugees Shami Ravo Hussain, left, and Gozi Haji, right, fled when ISIS came to their village Siba Sheikh Khidir, near Sinjar. Hide Caption 8 of 9 Photos: Fleeing ISIS: Life for Yazidi child refugees Gule Rasho, 23, and her son Rami, 11 months, also escaped from Siba Shekh Khidir.

Hide Caption 9 of 9

Ghazal Issa Omar sits with us on the floor of her tent. Alongside her are her two sons and their cousins. All four children were conscripted by ISIS during their raid on Sinjar. Under her gentle translation, the boys answer the questions as best they can.

Her 8-year-old son, Iman, has a shy gap-toothed smile.

The rifle he was given to carry hurt his arms, he says. But when he dropped it, the beatings would start.

Ghazal says the children were taken to the Badush prison in Mosul.

"They were taking them as shields. Raising them up high so that the airplanes could see them and wouldn't bomb them."

Iman's brother Assim is still only 10.

He tries to describe what it was like for them in the prison.

"When they took us to Mosul, to the Badush prison, they locked us in there," he says. "They treated us violently. I've never been beaten like this before. It was like dying."

A hope to identify the dead

"At night, when the planes came, that was when it was scariest. In the dark, we would huddle together. All us boys just holding on to each other."

None of them dared cry, though, he says, terrified of the beatings that would bring from their captors.

The day they managed to escape was the first time Assim allowed himself to cry. That was the day he finally saw his mother again.

Back at the outskirts of Sinjar, in the near distance, we can see smoke rising from a mortar strike into an ISIS encampment.

Mass graves honeycomb the valley leading to the boundary of their territory. On the ground, the mayor spots a fragment of what appears to be a child's skull. Delicately -- reverently -- he places it on top of the grave.

One day, he tells us, he hopes it will be safe enough here for forensic investigators to come and identify the children who lie beneath this this rubble.