This sounds really convincing but I'm not sure it's actually correct. The author is conflating the surprise of punchlines with their likelihood. To put it another way, ask a professional comedian to complete a joke with a punchline. It's very likely that they'll give you a funny surprising answer. I think the real explanation is that good jokes are actually extremely difficult. I have young children (4 and 6). Even 6…
In the adult would model there is absolutely no contradiction about the joke you mention - it’s just a bit of cute nonsense.
But in a child’s world this joke might be capturing the apparent contradiction - the sky is “in the tree”, so it must have climbed it, to be there (as they would have to do), yet they also know that the sun is already in the sky, so it had absolutely no reason to do that. Also, “because it’s already there” - which is a tricky idea in itself.
We take planetary systems and algebra and other things we can’t really perceive as granted, but a child model of the world is made of concrete objects that mostly need a surface to be on, so the sun is a bit of a conundrum in itself! (Speaking of my own experience remembering a shift from arithmetics to algebra when I was ~8).
If not too much of a personal question - I would love to hear what your child would answer to a question why she finds that joke funny. And whether she agrees with my explanation why it must be funny :-)