Well, I had one of those moments as a parent... you know, when you wonder "How am going to answer this question?" and after your best effort, the kid asks an even tougher one.
I told our 6 year old son that I would look for a (toy) helmet like a real Soldier wears. He's seen pictures of me in my 'battle-rattle' and he was asking me if I had extra uniforms since he knows that when my uniforms wear out, I give them to the kids or nephews to wear for purim costumes or to play in.
So he asks me if I can find an extra helmet, that no one needs. I patiently explained that every soldier needs their helmet, and there weren't any extra ones here. Then he says "do you remember a picture you sent..."
It was a picture of me officiating at a Memorial Ceremony for one of my Soldiers. In the picture, you can see the traditional display of a helmet on top of an inverted M-16 with ID tags and boots. I guess his mommy had explained to him that the Soldier "didn't make it." So now he is wondering if that helmet is available. I suppose it is, though it will be filled by another Soldier.
R. Shlomo Carlebach once said that while a Soldier hopes that when he pulls the trigger, his bullet will find it's mark, a Jewish soldier "is hoping that between the moment he pulls the trigger, and the moment the bullet hits, G-d will perform a miracle, and there will not have to be wars any more."
So, I patiently explained to our young child that yes, we lost a soldier, but that there are no extra helmets right now; and I pray that a miracle occurs and he will never need to wear one.