You could. But then it's still not the truth, the whole truth and nothing but the truth, is it?
Well they could explain the reasons why they are a c*nt without having to swear.
I don't think there are strong reasons to make up stories like the Santa story to tell to kids... it is even possible to be honest about sex with kids. See this video: (NSFW)
https://vimeo.com/27847934
You are kidding? This is how you think a six year old should be answered when they ask, "where did I come from"?
It reminds me of a story: Little Bobby came home from school one day and asked his parents where he came from. They sat him down and explained in detail all about sex. He sat wide-eyed and blank-faced until they finished then asked if he understood. To which he just shook his head and said, "Not really... Jimmy told me he was from Cleveland."