The year that I found out about the truth of Santa from my parents, i promised to keep it under my hat. However, I called them on the dishonesty of it all- no biggie, I just mentioned it.
A week or so later, my brother accidentally ran into the coffee table and broke something on it, and rather than go and own up to it, he went and tried to pretend he hadn't done it. He was confronted by my parents, I was in the room, and when my mom mentioned how lying is bad, my "this is hypocritical bullshit" detector went off, and I quickly blurted out, "YOU LIED ABOUT SANTA!" After a moment, my dad was dying with laughter, my mom was glaring at me with a hint of "I want to laugh but can't" and my brother was quickly piecing the puzzle together in his head, and looking very upset for the effort. Before he started wailing, I made up something about the milk we put out being put back in the refrigerator because it would go bad if it sat out, which he readily accepted.
Long story short,
I THEN got the story on "innocent,
little white lies", and not being disrespectful by undermining your parents directly in front of someone else- a "if you're going to call someone out, do it in private", sort of thing.
That being said, my imagination is one of my fondest personal posessions, and I thank my parents all the time for nurturing it to its potential. Seeing as allowing "Santa" (or whomever) doesn't permanently damage a child once they find out the truth, I've never seen harm in letting a child have something magical to believe in.
I think that's a large part of why there are a good number of people with no religious affiliation who nonetheless enjoy the festivities of the Santa-Christmas. You still get the good will, the "giving is better than receiving", the "helping the less fortunate", the presents (yay!) and the fun imagination of it all.
(Disclaimer- I don't have kids... I don't like kids... but i'm a kid at heart... take it for what it's worth.)
Oh, and, not for nothing- but some of you have literally written out, or heavily alluded to, your desires to keep your children young... so if "I want them to be kids as long as possible" is your reason, that's not really a healthy reason to not tell them, or do anything, in my opinion. That's just a selfish desire to keep them children, so you don't have to watch them grow up- which for some of you, annoyingly points out the progression of your own age, which portends their leaving the nest, which gives you a sense of "no longer being needed". Raise them right, and you'll always have their love- no need to keep the "you're a kid" strangle-hold on them. Just a point to ponder.