Why do people tell kids things that aren’t true? For example Santa Claus, the Easter Bunny, fairy tales, all that. It seems like until the age of 10 we unnecessarily live in a world that is half a fiction. At least later in life most sensible people acquire respect for the truth in the world, and accept fiction as being limited to books, films, music, poetry and other media.
If you do it right, you make it clearer and clearer to children that these things are a nonsense until they reach the point where they decide for themselves that they don't believe in them, and the earlier you can get them to that point, the better, but crucially, you mustn't tell them that they aren't real - you should tell them the opposite so that they are forced to disbelieve you. This is how you create strong critical thinking skills in children, generating intelligent ones rather than gullible fools who are in danger of swallowing every mind virus that comes their way.
When my younger brother was small, he would sometimes refuse to get out of bed. The only way to get him out of bed was to tell him there's a bus on the street. For some reason, he was excited to see buses. Sometimes, I was sent to get him up by telling him there's a bus out there, when there wasn't. Yeah, don't ask.