I think it was quite likely a slip of the pen, though it's also quite possible that Tolkien was reconsidering the fate of the Elves. I'm a bit wary about changing it. Anyway, "Children of Iluvatar" could refer to both Elves and Men, or it could refer just to Men. If, for example, I say "Elves", I could be referring to all Elves, or to some specific Elves, or to a group or type of Elves. The text as published, then, has the virtue of ambiguity.
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