How does he know that:
1) The children were killed? As opposed to playing dead for the cameras?
2) There were "hundreds"?
3) That any who were dead were from their "own side", as opposed to captured family members of the opposition. ISIS showed no hesitation in killing women and children.
That particular faction of rebels might have been on the outs with the faction that dominated in that village. There are so many family, clan, tribal, ethnic and ideological permutations, this guy is a prize ass just for venturing such a definitive opinion. He doesn’t know. Nobody really does.