We should teach children the horrors of war because it will prevent them from believing it's a viable option for every conflict. It will teach them that war and being a soldier is not all glory. It is blood and guts and metal issues. It will also help us enlist soldier who are willing to overcome these horrors.
No, children should not learn about the horrors of war. War is traumatic enough for the adults who have experienced it. Describing violent encounters of war would desensitize children to the affects of war and would encourage children to settle disputes through acts of violence. In short, adult situations should be left to adults!