However, after Abu Bakr al-Baghdadi declared IS' caliphate on June 29, 2014, more women traveled to Syria and Iraq with their children. In early September 2014, Meryam, her husband Jermaine and their three children (ages 2, 5 and 7) joined IS along with some other families from the Netherlands.

The London-based International Centre for the Study of Radicalisation and Political Violence estimates that at least 4,000 Western citizens have joined the Syria-Iraq conflict . About 550 are women who have migrated to IS-controlled territories, according to the Institute for Strategic Dialogue. How many Western children have been taken remains unknown. Most of the women travel without children.

The mother, a Chechen refugee , and their children disappeared on Oct. 27, 2014, from Maastricht, a city in the Netherlands, and arrived late December in Tell Abyad, a Syrian town near the Turkish border. They are now living in Raqqa, the self-proclaimed capital of IS. The father hasn’t heard anything from them for months.

"If my kids — if I ever get them again — become Muslims that would be fine by me. But only at an age when they are able to make their own choices with full awareness,” he wrote on March 22.

AMSTERDAM — For Aysha and Luca Opdam's father, it is still hard to accept that his children, born and raised in the Netherlands, are now living in Syria. In a letter, he describes how his ex-wife radicalized after she devoted her life to Islam around six years ago. After it was made public that Aysha, 7, and Luca, 8, were taken by their mother to join the Islamic State (IS), the letter was published by a local newspaper.

An anonymous source who knew the family described how the family lived according to the rules of orthodox Islam. Still, nobody expected them to travel to Syria.

"Meryam was always very concerned about the children. When they had a little scratch, she would go to the doctor immediately. It is awfully illogical that she has taken them to a war zone," said the source, adding, "Perhaps they believe that IS is invincible, or think life is worthless because paradise awaits them."

Women who have joined IS believe that Baghdadi has restored the caliphate and is the successor of the Prophet Muhammad. They see jihad as a religious duty for Muslims and are attracted to the idea of living in an area where Sharia is implemented. On social media they often ask their "sisters" in the West to perform hijrah, or immigration to a Muslim country.

"They do not go there to fight but to live what they call a purely Islamic life. Western jihadists believe that their children are better off in IS, because in Syria or Iraq they are not influenced by ‘nonbelievers,'" said Marion van San, a researcher focusing on radicalization and Western jihadists at Erasmus University. “However, some women decide to leave their children behind."

According to Abu Ibrahim Raqqawi, an activist with the group Raqqa is Being Slaughtered Silently, around 90% of the foreign boys over 7 go to an IS boot camp once they arrive in IS-controlled territories. In this military camp they undergo an indoctrination process and receive military training. Some of the children are also required to deliver food and medicine, fill the weapons depots and prepare bombs. Once they turn 16, IS sends them into battle, he told Al-Monitor.

"Their parents want them to fight to protect the caliphate in the future. IS doesn’t think about five or 10 years from now; IS is preparing to fight almost the entire world, the West included, for generations," Raqqawi explained.

In the latest issue of IS' online magazine Dabiq, the terror group also hints that it is engaging in long-term planning, explaining that those who are capable are allowed to participate in operations against the "mushrikeen," or polytheists.

“As the mujahideen of IS continue their march against the forces of the kuffar [non-believers], there is a new generation in the wings," reads an article alongside a picture of the young boy who appeared in a video shooting dead an Israeli Arab accused by IS of being a spy.

IS has recruited at least 400 children in Syria in the past three months, according to the Syrian Observatory for Human Rights. But the "Cubs of the Caliphate" are also being indoctrinated in other ways. For example, they may watch violent videos and attend public beheadings.

On social media, Western female jihadists constantly post pictures of children with IS flags, guns and knives. Relatives of European women who joined IS point out that the women glorify violence and that they have adapted to their new way of life quickly. One told Al-Monitor, "When I talk to my relative, she only tells me good things about IS. One day, she shamelessly said that her Western friend — who also joined IS — signed her 10-year-old son up for a military camp. A few months ago, this boy had a normal life. He went to school in Europe, where he had friends."

However, not all foreign children undergo military training immediately. Recently, IS opened a new kindergarten for boys in Raqqa, and there are still schools where children attend Sharia and Arabic classes. As for the girls, they mostly stay inside the home or go to girls-only schools.

It is difficult to determine how Western children are coping with their new environment. A Syrian resident in Raqqa told Al-Monitor over Skype, "Sometimes I see the young kids of foreign fighters play outside. They seem happy, but aren’t kids happy almost all the time and everywhere?"

While media outlets mainly focus on young Western girls marrying jihadists, many Western women arrived in Raqqa with their children, with or without their fathers, said Raqqawi. He added that Syrians find it absurd that "Western women take their children to a war zone while so many locals are desperately trying to get out of the country."

Van San stated that the violence doesn’t scare off radical Western women because they are convinced they will go to paradise if they are killed. "They believe that they and their loves ones will be reunited in paradise," she said, citing a recent interview she conducted with a woman whose son died fighting for IS. “She said, ‘If you have the option to choose between living with your son for 30 years or living the eternal life with him in [paradise], what would you choose?' For them, it is that simple.”

However, those who personally know women and children who have joined IS find it hard to accept.

"Maybe it is better if Meryam and her husband die before they commit a war crime. I only hope their children will come back, but I don’t think this will happen. And if the kids come back, they must be damaged for life. They have been exposed to so much violence," said the source.