Yes, a child at war is not there out of his own accord. He has been coerced and manipulated to be there. Thus, he can't be a true fighter. He is a victim. In no way should a child ever be beheaded. That is wrong on every level. It makes me very sad.
The Liwa al-Quids could have easily picked a small boy off the street and used him as an example. But it is always possible that he could be a fighter as there are fighters this young in that region of the world. However, it is always possible that they simply picked him off the street.
It is my opinion that sadly, in war torn regions such as Syria is currently, children are often recruited to fight in armies, such as this 12 year old boy beheaded by Syrian rebels. During wartime the most basic structures of a society, such as schools, can crumble, leaving adolescents with little choice but to take up arms and fight.
The use of child soldiers is nothing new in combat. They have been a staple of despotic and militaristic regimes in the third world for the psychological effect they have on those they fight against. The thought of killing a child, even in self defense can be a traumatic one for those who face it. That said, should this child have been beheaded? No.