There's only one source to my knowledge for this supposed third child, a letter written to a Florida paper which didn't publish it. Freepers have been in contact with the writer, but I know next to nothing about that. I don't give it all that much credence. It's the kind of thing that takes on a life of its own. Because one person alleges it, it must be true and therefore becomes grist for the mill.
Until and unless it is verified, like some of the other things alleged in this situation, I tend to dismiss it.