Christmas Special and famous editorial

[Alien1992]

hi guys
I hope you'll help me to find the answer. I can't understand the following thing. It is the dialogue between Mr. Foley and Mr. Willoughby:
Mr Foley: 'Excuse me, Dr. Willoughby. You busy?'
Mr. Willoughby: 'George. I mean, Santa.'
Mr Foley: 'I just thought I'd stop by to wish you a Merry Christmas and to give you this for the hospital.''It's not much.'
Mr. Willoughby: 'Well, thank you, George. Thank you very much. Marry Christmas to you too.''Where should I send your receipt?'
Mr. Foley: 'Don't bother. Don't open it until tomorrow'
Mr. Willoughby: 'Did you give out all those presents?'
Mr. Foley: 'Well, all except for that oddball.''Tiffany took it, and then she gave it back.'
Mr. Willoughby: 'Well, all she really wanted was to see you for Christmas.''One of the other kids told her there's no such thing as Santa. Then she asked me if it was true.'
Mr. Foley: 'Well, what did you tell her?'
Mr. Willoughby: 'Well, I showed her this.'
Mr. Foley: "Yes, Virginia, there is a Santa Claus."
Mr. Willoughby: 'The most famous newspaper editorial of all time.'
Mr. Foley: "He exists as certainly as love and generosity and devotion exist. How dreary would be the world if there were no Santa Claus. It would be as dreary as if there were no Virginias. There would be no childlike faith then, no poetry, no romance to make tolerable this existence. The eternal light with which childhood fills the world would be extinguished."
Mr. Foley: 'Tiffany?'
Mr. Willoughby: 'I can't do anything for her.' 'What are you supposed to say to a girl who's not gonna see another Christmas?'
End of the scene.
I don't get how Mr. Foley guessed that Tiffany was terminally ill. By reading an article?
I've read both Francis Pharcellus Church's article and Virginia O'Hanlon's letter. The article was written in terms of idealism. Аuthor declares the primacy of idea in relation to matter and touches upon the issues of being, faith, importance of creativity and fantasy in human's life.
Sorry if I said sth stupid, I'm not a philosopher.
So after reading this article, in which complicated things are explained by simple language, a child should understand the importance of faith in good. And if there is no good at all, it's up to everybody to create it, first in our minds, then in real.
It is a great article, isn't it!
But I still can't understand Mr. Foley's logic.
All right, I thought, maybe Virginia O'Hanlon had a fate similar to Tiffany's? She was ill and died in her childhood? Fortunately, no. Wikipedia states the following: "Douglas died on May 13, 1971, at the age of 81, in a nursing home in Valatie, New York." Douglas was her husband's last name. She was a school teacher, by the way.
To sum it up, i dunno how Mr. Foley guessed about Tiffany, if i were him, i couldn't.
Either I am dumb or I don't get it because I'm not an American.
I humbly ask to share your thoughts about it.