What does the officer think the boys have been doing?

He thinks they are playing a game out of a storybook. Treasure Island....

Also,

"What have you been doing? Having a war or something?" His question is unintentionally ironic; kids play war, cops and robbers, armies and soldiers all of the time, and his question is referring to that, and it is a gross understatement of the real war that was going on. Ch 12