This is reprehensible, and disgusting. You do NOT train to shoot children, no matter the situation. If I saw someone shoot at paper targets with pictures of children without hesitation or protest, I would assume psychopathic tendencies. If I were in a profession - military, police, whatever - where I was asked to shoot at these targets I would seek to terminate my employment immediately and begin some sort of protest process.
I would rather be shot and killed by a child than to kill one, even in cases of self-defense. I used to be a kid, and therefore can say with experience that kids are dumb. They lack life experience. I can personally recall many, many things I did as a stupid youth that probably should have killed me dead. I was fortunate to survive long enough to get wise to how the world works. When you shoot and kill a kid, you're shooting and killing an innocent who really doesn't know better.
In the field of warfare, morality is shifted, because the conditions are also shifted. That kid with the gun is no longer a personal problem, he's an enemy combatant that threatens not you, but everyone around you. You may be willing, as I am, to die rather than to shoot a child, but are you going to hesitate to shoot him (or her) when he/she is threatening to kill everyone around you?
My answer is this: Deal with this problem if/when it occurs. DO NOT TRAIN TO SHOOT CHILDREN (who could be holding toy guns) ON SIGHT!!! Why on earth would you train to desensitize yourself against shooting children?? This sort of thing should ALWAYS be a difficult decision one makes in a difficult situation, NOT something you actively train for. Historically speaking, if you find yourself fighting a war in which children and pregnant women are shooting at you, (ahem, Vietnam) it's probably time to rethink the reason you're there. How did it come to this? How did it come to a situation where I'm shooting at expectant mothers and children?
Let me be clear - I would shoot a pregnant woman or child if they ever demonstrated that they were about to kill my own children or loved ones and there was no other way to stop it. It would haunt me to the end of my days and provide plenty of psychological damage. BUT YOU DON'T TRAIN FOR THIS! YOU DON'T DESENSITIZE YOURSELF TO THIS! Because one day - mark my words - someone WILL kill an innocent due to this frakked-up training method. Someone, after spending hours at a shooting range putting holes in pictures of children and expectant mothers, will put real holes in real people that may or may not be actual threats, but they put holes in them anyway because they were trained to. This sort of thing should be the unfortunate, horrible choice that one has to make in the field - not something that becomes standard operating procedure.