Grim, I have not the faintest idea what that clue (if such it be) was about, but here is my answer: the Creator made the addition, which was elves and men.
Letters, #212
Elves and Men were called the 'children' of God', because they were, so to speak, a private addition to the Design, by the Creator, and one in which the Valar had no part.
Letters, #212