From my reading of it, the father was responding in kind to the son's level of comprehension, his son's level of incredulity and at what the son in particular found amusing about that creation story. I didn't read imposition unless one confuses his narrative to the reader with what he did/didn't say to his son.
With the father laughing and starting his story line with "tell him about the lie", do you really think this young child did not pick up on his father's amusement with the idea? You know as well as I do that how he presented it made more of an impression than what he did or didn't say.
I'm not saying an atheist cannot tell his child he thinks it's a lie, but to have me believe that a six year old decided all by himself that it was ridiculous and God must be 'an idiot' for making wars doesn't hold up. And if he laughed at the idea of the world being small, then what did dear old dad use to explain the Big Bang? First there was nothing and it exploded?
Everyone has the right to tell their children whatever they want to. Just don't be surprised if that is what they accept.
Please read it again. The "tell him the lie" was part of the narrative to the reader, not how he started the story that he read to his son.