I'm an atheist and I tell my children adults invented Santa Claus so children will behave and go to bed early in the winter while the adults get drunk and fornicate with each other.
In fact, I instruct my children not to lie and to tell others the same thing if asked about it. I don't lie to my kids.
Now if you think that is bad, lie to your kids all you like and they will lie to you.
I don't want my kids to lie to me, and they expect the same from me...
Lots of ugly things are called "truths", and I don't think people are supposed to go around saying ugly things to each other, and especially not to children. Or don't you believe in politeness? We are supposed to treat each other with kindness and love. Which means if someone is retarded, I will not tell them they are "stupid", if they are deformed, I will not call them "ugly", if someone is large, I will not call them "fat". And it is most certainly not my place to ruin Christmas for a bunch of children. It's a shame you are teaching your children to be rude.