This is an amazing thread.
Here's another route for establishing the age, longevity and continuity of child (and other) ballads in America. It seems that 28 of the 39 singers whom Cecil Sharpe collected from in Madison County NC traced their descent
from the same man, one Roderick Shelton, first settler in Shelton Laurel. By the time Sharp was collecting, the singers had their own variants (and subsets) of what seems to have been one ancestor's repertoire. (this comes from the article 'a nest of singing birds' by mike Yates and kriss sands at the MT website and they cite Betty smith's use of research by Frances Dunham in the Jane hicks Gentry biography.) ... Perhaps someone even has a way to calculate time from the divergences, as in evolutionary biology. Though that isn't really necessary, because some singers today can say that they are eighth-generation ballad singers.