The Canadian senator, now 69, was commander of the United Nations Assistance Mission for Rwanda (UNAMIR) in 1994 - and his inability to secure UN intervention to stop the genocide which killed 800,000 people has haunted him ever since. He was medically discharged from the army with PTSD and in 2007 he launched The Romeo Dallaire Child Soldiers Initiative, with a mission to stop the recruitment and use of child soldiers worldwide.

And so, last month, the retired soldier was invited to attend a Toronto Film Festival screening of the film, which tells a story so close to his heart.

“It’s the classic Blood Diamond story of disaster in Africa,” he said, having seen the film. “But it doesn’t give an analysis of the situation. There was a lot missing.”

It is however, he willingly admits, a gripping tale.

Directed by Cary Fukunaga, whose previous work includes Jane Eyre, season one of True Detective, and intense Central American migrant drama Sin Nombre, Beasts of No Nation tells the story of Agu, the 14-year-old boy whose life is turned upside down when he loses his family in tribal violence, and is recruited to become a child soldier.

It is based on the 2005 book of the same name by Nigerian author Uzodinma Iweala, and set in a nameless West African country.

“What drew me to it was the perspective; seeing it from a child’s point of view,” said Mandela star Elba, a Londoner whose mother Eve was born in Ghana. “The whole concept of child soldiers – people know about it, but not in detail.”

The film was shot on location in the jungles of Ghana during monsoon season, where Elba had to call in all the favours he had to find props and loan vehicles. Resources were scarce, floodwaters wreaked havoc with the sets, cars were ambushed and extras imprisoned. The director caught malaria and Elba fell off a cliff, only surviving by clinging onto a tree overhanging the waterfall.

The film is made by Netflix, which will for the first time show the movie in cinemas and offer it online. It is being seen as a watershed moment for the film industry, which could transform our attitudes towards cinemas and home viewing.

“It’s been an amazing journey,” said Elba, speaking at the film festival. “I’m so proud of what it’s done, and how it’s been received.”

He produced the film and dominates the screen as Commandant – the warlord who controls the boys. It is a role which is already generating Oscar buzz, and garnering rave reviews.

Even more talk has been of Abraham Attah, a first-time actor who was chosen to play the lead role at an open audition, and stars as Agu.

The critics have been falling over themselves to praise the vivid, intense and atmospheric drama.

Lt Gen Dallaire is somewhat more reserved, however.

“I’m not against the film,” he said. “Cinema is an extraordinary tool. But it really didn’t tell the whole story.”

So what is the whole story?

The charity War Child estimates that there are around 250,000 child soldiers worldwide – a child being under 18.

The assumption remains that it is an African problem, but Lt Gen Dallaire is at pains to point out that India, Colombia, Thailand and Burma are just some of the countries known to have problems with rebel groups using children.

Ben Affleck, Abraham Atta, Cary Fukunaga, John Legend and Ted Sarandos at the ‘Beasts of No Nation’ film premiere in Los Angeles  Photo: Rex

“We were working in Sierra Leone, alongside some British soldiers,” he said. “And when they heard us deliver our training to the Sierra Leonean army on how to handle a situation involving child soldiers, one turned to me and said: ‘Where the hell were you when we were facing this in Afghanistan?’”

Military man that he is, Lt Gen Dallaire approaches the issue on several fronts: training troops in how to deal with children on the battlefield, carrying out research into the scale of the problem, and working to raise awareness of the fact that having children among your ranks is not an asset.

“You have to explain how to handle it,” he said, with classic military mentality. “If you attack them, you are committing a crime against a child. And if you turn and flee, you are not achieving your mission.”

This year he and his team have been working extensively in Uganda, training 20 generals and other senior military commanders in how to cope with child soldiers. They then returned, so that those they had trained could pass on the training to 100 more. Later this year they will travel to Ethiopia to rewrite the constitution of the African Union, as they did with Nato, to include measures for dealing with child combatants.

And he has recently returned from Jordan, where, he said, the refugee camps can become ripe recruiting grounds for Islamists to pluck child soldiers.

“I think there is a dearth of information in particular about Islamist extremists and the use of child soldiers,” he said. “People are edgy about bringing it to the floor. They worry about going into something so complex.”

But groups such as al-Shabaab and Boko Haram were making extensive use of child soldiers, he said. And it is not something that Western forces can shrug off, saying it is something they will never come into contact with.

Idris Elba in “Beasts of No Nation”   Photo: Netflix via AP

South Sudan – to where Britain is poised to send 300 peacekeepers – is one of the countries in which he works.

“I think the film could certainly have done more to show how most peacekeepers are really not trained,” he said. “That’s at the heart of our work.

“I think the film could have done more to show the indoctrination of the children, and the psychological battles. It needs to be more nuanced than just African kids with AK47s.”

Helen Morton, director of advocacy for War Child, agreed.

“Forty per cent of child soldiers are girls, and few films ever portray that,” she said. “Girls are combatants - and in growing numbers. They are forced to do things that are beyond even a child’s imagination, and often recruited as sex slaves.”

Just a few weeks ago, 163 child soldiers were released in the Central African Republic, she said, and War Child is working to reintegrate them, and a further 3,000, back into their communities.

The UN is making real progress on the issue, she said, but more must be done.

“If the film shines a light on the issue then it’s certainly useful. But it’s important that audiences realise it’s not fiction - it’s fact. And as horrifying as some of the scenes are, they are more muted than the reality facing 250,000 child soldiers on a daily basis.”

Has any film ever got it right?

He paused. “American Sniper was getting there,” said Lt Gen Dallaire. “It did deal with the dilemmas of children becoming Islamist extremists. But there was no mention of how the kids were indoctrinated and nurtured over years.

“There’s a good film called Hyena Road, set in Afghanistan, which does well. And documentaries are getting there. People tend to forget that it is through the recruitment of child soldiers that conflict itself is sustained.

“But look – it is crucial to talk about genocide prevention, and mass atrocities. So regardless of my concerns, I do think people should go and see it.”