Some do it simply as a matter of survival.
Those are some of the motives explored in the compelling documentary "Fight Like Soldiers, Die Like Children," which is framed through the experiences of Sen. Romeo Dallaire, an ardent advocate against the recruitment of children for the battlefield.
Dallaire knows the horrors all too well. A longtime soldier with the Canadian Army, he commanded a United Nations force that tried in vain to stop the 1994 Rwandan genocide.
In the film, he laments that the sheer number of youth available to rebel armies makes them expendable. He describes the roughly 300,000 child soldiers fighting worldwide as "the most sophisticated low-technology weapons system in conflict today."
Dallaire jumped at the chance to do the new film with director Patrick Reed and producer Peter Raymont, whom he had also worked with on a previous documentary based on his book "Shake Hands With the Devil."
"When I decided to dedicate the rest of my life to finding a way to end the use of child soldiers, I knew that I had to go back to Africa and face my ghosts again," Dallaire says in a statement.
The film, which has been shown in other parts of Canada, is being released to theatres in Quebec on Friday, the same day screenings are scheduled in Calgary and Winnipeg. It is also being shown in Victoria starting June 16.
Former child soldiers are interviewed in the film, as are officials working to end the phenomenon.
The most gripping moments include conversations with a military man whom Dallaire confronts about their use of child soldiers.
They dodge the subject and deny ever using them. Otherwise, they know they could face prosecution.
Dallaire speaks with disdain of those who exploit children as soldiers.
"The use of children as a weapon of war should be as abhorrent as using nuclear weapons," he says in the film, calling the act "a sin."
The military man's attitude didn't surprise anyone on the documentary crew.
"Most of the commanders know what they're doing, particularly of there's an outsider there or anybody with a camera," Reed said.
"They know if they're caught in the act, it will probably not help them in the long run. They're very smart in the sense that they'll keep the kids behind the front lines and also they'll hide their child soldiers except in rare circumstances."
Reed says most people think of the brutalized and abducted combatants of the notorious Lord's Resistance Army operating in northern Uganda and South Sudan since the 1990s when they think of child soldiers.
He says they're not as typical as believed.
For example, some children like the 14- and 15-year-olds in Eastern Congo may be forced to choose between hoisting an AK-47 in a rebel army to defend their village or just standing by and watching their community be destroyed.
"They're placed in a kind of choiceless choice," Reed explains in a telephone interview.
"In a lot of cases, those kids don't have many other choices and there's no jobs," he said. "Having a gun gives them a sense of power and prestige, for better or worse."
Then there's the so-called Arrow Boys of South Sudan.
The ragtag group equips itself with primitive weapons, such as bows and arrows or even homemade guns, to defend members' homes and families against marauders who would drag their relatives into war or slavery, Reed says
"They're in an area where there's a huge UN base just down the road and basically all the UN soldiers are staying in the base and not patrolling and protecting the locals," Reed said.
"The Arrow Boys are like a community defence network."
He said he saw teenagers going out on patrol with the Arrow Boys as well.
"As a westerner, you can sit in judgment of that and say obviously that's not ideal but, in the absence of any international or national force that's protecting people, I don't know how we can sit in judgment. It's a complex situation."
The film also examines the daunting task of helping demobilized children recover from being ordered to kill or live as sex slaves, the latter often producing children with rebel commanders.
Reed says he was surprised to learn that about 40 per cent of child soldiers are girls who usually do logistics or serve as so-called "bush wives."
They face a particular challenge being reintegrated into their communities afterward. They are usually stigmatized because they've been raped or had children by their captors.
Reed said while the film was logistically challenging to make, he enjoyed working with Dallaire, who seemed to be relieved to be out in the field and away from the political bubble in Ottawa.
"He's a very intense guy," Reed said of the Liberal senator.
He recalled Dallaire did present some security concerns because of his fame but that notoriety helped a lot in other cases, especially in negotiating a particularly difficult border point.
"There's some UN guy with a .50-calibre gun on the back of his truck with a little sign that says 'General Dallaire'," Reed recalled.
"It felt like we were being picked up at the airport by a limo driver."