This is such a recent discovery that I cannot think of more clues. Two years ago I received an email from James Cornelius telling me of a new find. The real author of "Little Eddie" had been discovered. She was a young girl whose name was Mary E. Chamberlain, but she was using the pseudonym Ethel Grey.
The whole story is online. Please see the article by Sam Wheeler entitled "Solving a Lincoln Literary Mystery: ‘Little Eddie’" here.
Wow, what a find IMO! Thanks for bringing this up, Roger! Very interesting also how it comes that false assumptions (the parents wrote it) become popular belief.
Correct, Gene. Very good! I believe nowadays the original letter Grace wrote to Abraham is in the Burton Historical Collection of the Detroit Public Library.