If I read it correctly.
A) he only wants to kill me, no one else
B) I can't kill him without killing the child (conversely I can't miss the child without missing him)
C) I am fairly unlikely to be hit from that distance
Therefore, I am not in any imminent danger, neither is the child unless I shoot. If I disengage, everyone lives. If I engage, the child dies. I am 100% responsible in this situation.