Publishers Weekly has an article about how some publishers are treating war in children's books, including picture books. One of the main comments from the article seems to be this:

“I don't think you can change the truth of a story to make it have a happy ending if it doesn't have one, but I think that for a picture book audience it needs to have a narrative arc,” said Allyn Johnston, publisher of Beach Lane Books....“It's really the story of one girl and her devoted grandmother, and what she's doing to try to bring her granddaughter back into the world after this loss. But then, as much in the images as in the text, you get to see the wider situation of what's happening there.”

This may be applicable to other tough problem books as well. The article gives titles and brief synopses of about 20 fiction, non-fiction, graphic novel, picture and allegorical books related to war.