Whether someone had chucked a grenade or not is irrelevant. There is no reason to think that any of the boys being kicked around had thrown it.

And if they had picked up a genuine insurgent rather than a random teenager, that's not an acceptible way to treat a prisoner of war.

As haas been pointed out in press covereage here, these soldiers have increased the danger to other soldiers, and to other people as well. They were, paradoxically enough, "giving aid and comfort to the enemy".