Antwerp, Belgium (CNN) An 11-year-old at the back of the class raises his hand and proudly delivers an answer to the teacher's question: "If you murder one person, it's as if you killed all humanity."

It is a verse from the Quran, delivered in Flemish.

And for the teacher, Ibrahim's answer is exactly right.

Curly-haired Ibrahim is one of almost 100 children who fill classrooms in this Belgian city after school, attending "Muslim Values" classes.

Next month, when they move to a bigger building, that number will rise to 300.

Front line against radicalization

Belgium has emerged as ground zero in the fight against radical Islamic thought in Europe.

Several of the Paris attack suspects had links to the country, and per capita, Belgium is among the biggest providers of jihadi fighters to the ranks of ISIS.

ISIS, Belgian authorities believe, is recruiting with techniques that mimic those used by cults. And it is targeting an alarmingly young demographic.

Children as young as 9 and 10 are being exposed to jihadist thought and the ISIS worldview, authorities say. So classrooms like imam Suleyman Van Ael's are a new front line.

Van Ael converted to Islam 20 years ago. His mother had converted when he was a young teen and she was living in Dubai, a change of faith that upset Van Ael at the time. But late in his teens, he began reading the Quran, and when he was 19, he converted, too.

Now, he's delivering what he calls a "vaccine against radicalization," teaching his students that the attacks on Paris were "un-Islamic."

ISIS targeting children

He set up these after-school classes to provide an alternative to the radical messages the children are hearing.

Imam Suleyman Van Ael aims to deliver a "vaccine against radicalization."

He's made it his life's work to unpick the radical ideologies and influences to which some Muslim children are being exposed.

"The detailed information that they have is for me the most disturbing," he said, such as children asking about the death toll and violence in Syria or the after-effects of the U.S. invasion of Iraq -- essentially all ISIS talking points.

"Because when you're 8 years old, you should just enjoy life. Go to the park. Go to your mummy and eat some cake, and that's it," he said.

Much of the messaging that is bombarding the children, Van Ael says, is online.

"They are exposed to YouTube videos, and from one video, you could go to another, and in the end, you find yourself getting information that you shouldn't get at that age," he warned.

Some ISIS propaganda has very clearly targeted children.

The "Cubs of the Caliphate" videos depict child soldiers training in ISIS garrisons, handling weapons and even carrying out executions.

Family ties

Compounding the issue, authorities say, is the hero worship that can be attached to young men in the Belgian Muslim community who have traveled to Syria to fight.

Some of them have younger siblings in Van Ael's classes.

The suspected organizer of the Paris attacks, Belgian Abdelhamid Abaaoud, bragged of getting his 13-year-old brother to follow him to Syria. The younger Abaaoud is thought to be the youngest foreign jihadi recruit to ISIS.

So how do you stop a child wanting to follow in an older brother's footsteps?

"What we try to do in general is to make a distinction between 'This is your older brother who you love' and 'These are the deeds of your brother,' which doesn't mean that you should not love your brother, but you should not love what he is doing," the imam explained.

In the wake of the Paris attacks, these classes have taken on a renewed urgency.

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The discovery of Belgian residents and nationals at the heart of the plot reminds everyone here of how much is at stake.

The imam does not excuse the Muslim community from some responsibility.

"We shouldn't be too afraid to point the fingers at ourselves and to say that we also have work to do to change this kind of violent radicalization," Van Ael said.

His work does not come without a price. Van Ael says he has been subjected to a number of ISIS threats, none of which he wishes to detail.

In the beginning, he admits, it got to him: "I stopped teaching."

But he came back to it.

"It's not something that is easy, but in the end, you say if your goal is good, then it's worth going for, and whatever happens, happens. It's not only about my life."

And, he says, he is not alone in his mission: "If I'm erased, there will be someone who thinks like me and will keep on going."