How does the killing of an innocent child ever become moral?
You are arguing from a faulty premise.
When God says it is. You might remember a few instances where He says to kill every man, woman, and child in a given city. I suppose God's a war criminal in your eyes, too, right? My Soldiers blasted a 12-year old boy in Iraq. He opened up on us with an AK47, handed to him by his Al Qaeda buddies. I distinctly remember looking down at his bloody corpse, trying to muster up SOME sympathy, but utterly failing. I guess I'm a war criminal, too, right?
Moreover, do you know what the invasion plan for Japan was? It involved nerve gassing millions of people (the estimated casualties were to be around three million). And that was before one Soldier stepped foot on the ground, not counting any other air raids or naval bombardment in preparation for the invasion.
How should the war have been concluded, to your view?