"You broke my child's heart!" No, actually, you did that.
When I was but a wee bitchy tyke, I figured out that Santa didn't exist. It was not a fun time for me. I know that there are many people who figured out the non-existence of fantasy figures without material pain or distress. Unfortunately, I was not of that personality. I started crying and it was a long time before I could be consoled. It shook my faith in my mother and aunt and all the other people who told or implied to me that Santa was real.
No doubt, many people will say, "Well it's just a bit of fun! It's an integral part of childhood! Don't take it so seriously!" Well, when you're seven, you DO take it that seriously. Your parents/guardians are your entire world and what they teach you, you live by. When you find out that your beliefs about something are nothing more than a series of lies told to you by the people you are supposed to trust the most, it makes you wonder what else they are lying about.
My mother didn't intend to hurt me. She raised me the best she could and she wanted me to have all the happinesses and "magic" that childhood was supposed to come with. She didn't think - and a lot of parents don't - how it seems to a kid who has been raised with such exhortations as "Don't lie!" and "Be honest!" to find out that their parent(s) have been lying to them all along. She also didn't realize just how much I depended on that world of fantasy and how much I believed in it.
I find it painfully amusing how some people safeguard their lies with righteous wrath should anyone happen to mention that they buy presents or make Easter baskets or provide the money for the tooth fairy. Or if someone posts something on the Internet suggesting that their constructs are not real. Suddenly, this person "ruined" Christmas or Easter or whatever holiday. The parent never takes the blame for creating a fantasy world for their child(ren) that must inevitably be shattered.
Why don't more people teach their kids the concept of "the spirit of Christmas" or "we pretend that the Easter bunny comes to bring you candy", they create a fantasy world that can be shattered with a single reference to the truth. Better, it's a lot easier to explain why some kids get a lot of presents and some kids get little or none.
No doubt there are a lot of kids who took the whole Santa/Easter Bunny thing in stride and, no doubt, for them it wasn't a big deal. But how do you know if your child will take it badly? I'm not suggesting that making a kid believe in Santa Claus and the Easter Bunny will turn them into a heartless killer. I am, however, putting it forward that to do this risks hurting a child deeply if they really, truly believe, only to be told the truth on the playground or overhearing a casual conversation. What I don't understand is why anyone would risk hurting their kid like this.
No matter how much people want to create "magic" for their children, I think a lie is still a lie. And it's just as hurtful as any other lie. Dressing it up in "magic" doesn't change it.