The day your kid discovers Santa Claus isn't real you take him out for dinner with family and friends, explain the lesson and give him a badge or a bicycle or something. Welcome him to the next step on the path to adulthood.
That sounds like something I'd want to do for my kids (provided I have any,) but what if instead of discovering it through their own reasoning or investigative abilities, they hear it from someone else, and come begging you to reassure them that it's not true?
I think if they wanted reassurance I'd tell them to figure it out for themselves, and possibly use it to teach them the concept behind the Litany of Tarski. How much that would work would depend on how old they are. After they realized it wasn't true and accepted that, I'd take them out to dinner.
I've long entertained a dubious regard for the practice of lying to children about the existence of Santa Claus. Parents might claim that it serves to make children's lives more magical and exciting, but as a general rule, children are adequately equipped to create fantasies of their own without their parents' intervention. The two reasons I suspect rest at the bottom line are adherence to tradition, and finding it cute to see one's children believing ridiculous things.
Personally, I considered this to be a rather indecent way to treat one's own children, and have sometimes wondered whether a large proportion of conspiracy theorists owe their origins to the realization that practically all the adults in the country really are conspiring to deceive children for no tangible benefit. However, since I began frequenting this site, I've been exposed to the alternate viewpoint that this realization may be good for developing rationalists, because it provides children with the experience of discovering that they hold beliefs which are wrong and absurd, and that they must reject them.
So, how did the Santa deception affect you personally? How do you think your life might have been different without it? If your parents didn't do it to you, what are your impressions on the experience of not being lied to when most other children are?
Also, I promise to upvote anyone who links to an easy to register for community of conspiracy theorists where they would not be averse to being asked the same question.