I know it's hardly folk, but Eric Clapton's "Tears in Heaven" does me in every time, thinking about his little boy who died.
Other than that, "There were roses," and a Yiddish song, "S'dremlen feygl" which was a poem written by Leah Rudzinski after a Polish ghetto was "cleansed"- there were few survivors, among them a 3-year old for whom the lullabye was written. Rudzinski later disappeared before WWII was over.
Allison