I think this poster is very well done and it conveys the point, but I feel it's conveying it a bit too strongly. Overdoing it, would be the word. The kids all bleeding, him crying. I think trying too hard to make a point might start causing doubts on how much you really have to say.
On a visual design point of view, nice poster, even if a bit too generic and "done before". It feels like this type of sad image+same-ish font+some tears has been done one too many times.
That said, I am very looking forward to seeing the film, as I've said before. The trailers are awesome, the poster is good and frankly, who gives a damn about those when the cast is so kick ass? Ok, promotional material can make or break a work, but still, you're very forgiving if you know what you're getting into.
And despite it being a bit lacking, the poster does have some nice elements. The kids are there, which is quite important and I do love Gong Yoo's look. Like I said, he can lighten your soul with one gaze and sink it in despair with the next.