When is it OK to shoot a child soldier?

I think the closing line sums it up nicely. Killing children is not, and will never be, OK. It's a truly terrible thing to do, and only a truly monstrous person would ever do it without remorse. But war does, sometimes, force you to do terrible things, and you have to live with that for the rest of your life.

This seems to me like a fine attempt at, at least, saying "we understand".

Of course, many people will struggle in the aftermath regardless of how necessary their actions may have been. "Oh I had to kill that kid who could have been my nephew..." can be very cold comfort indeed. In the worst case when someone is trying to kill you with a blade you're going to shoot them, and it's probably going to be pretty traumatic.

Doing otherwise would not only put you and your own troops in danger; it would also encourage ruthless and immoral warlords to use more child soldiers, since they would have such an advantage.

...but the short answer: It's never OK, but perhaps a necessary evil if you're in the me vs them situation. But that should go for adults as well in general.