Sam and Dean were young, something like 9 and 13, and they were kidnapped by a human and buried underground for I think longer than a few weeks. It focused a lot on their time when they were trapped and had limited food. They got more intimate then, too, and when they're found eventually both are very weak and thin. Sam gets nightmares a lot. I think there was a sequel where John takes them to a remote cabin so they can rest. I can't seem to find it in my bookmarks- does anyone know the story I'm talking about?