Some are as young as five, and in their hands are not toys but weapons of death.
Their minds are not on images like cartoon characters and toys, but military fatigues and blood.
These are the child soldiers of the Islamic State in Iraq and Syria (ISIS).
"I have met child soldiers once or twice," Mr Abu Mohammed, a resident of Mosul, Iraq, told USA Today.
"One of those soldiers was only five years old and carrying a weapon. I was shocked."
Shocking videos show adult trainers beating children with sticks and forcing them to break concrete blocks over their heads during martial arts training.
They are also shown being taught how to detain prisoners.
Some 600 children younger than the age of 13 patrol the streets of Mosul, an activist group reported.
Most are said to be children of ISIS fighters.
"They have become a necessity," Mr Riyadh Mohammed, an Iraqi journalist and former justice ministry spokesman told USA Today.
This is because the terror group controls large areas of Iraq and Syria and needs more fighters to expand.
Terrorism expert Ryan Mauro told news.com.au that ISIS is emphasising child recruits not only for the publicity but to show that it has multigenerational fighters.
"You can kill off the current leaders and fighters, but their kids will fight on. It makes it harder to celebrate (ISIS') losses if you know their manpower will be replenished with brainwashed children," he said.
A video that has emerged from Kazakhstan is even more disturbing.
It shows boys wearing matching camouflage military fatigues watching another boy strip apart a machine gun.
The narrator in the video then says: "They will move on to do physical and military training until they get older and pound the disbelievers.
"They are the next generation. They are the ones who will shake the earth...''
The child soldiers are then shown carrying out gun manoeuvres and tactical formations, reported the Daily Mail.
It also briefly shows shots of the boys working out in the gym and practising martial arts.
When asked what he plans to do with all this training, the boy says with a smile: "I will be the one who slaughters you, O kuffar (non-believer).''