The link between the scientific and the fantastic also explains why children's fantasy demands the strictest logic, consistency, and attention to detail. A fantasy without that logic is just a mess. The effectiveness of the great children's books comes from the combination of wildly imaginative premises and strictly consistent and logical conclusions from those premises. It is no wonder that the greatest children's fantasists—Carroll, Lewis, Tolkien—had day jobs in the driest reaches of logic and philology.
Still, we might ask, why do children explore the far and fantastic possible words instead of the close-by sensible ones? The difference between adults and children is that for most adults, most of the time, imagination is constrained by probability and practicality. When we adults use our everyday theories to create possible worlds, we restrict ourselves to the worlds that are likely and the worlds that are useful. When we adults create a possible world, we are usually considering whether we should move in there and figuring out how we can drag all our furniture with us.
But for human children, those practical requirements are suspended, just as the jungle laws of tooth and claw are suspended for young wolves. Children are as free to consider the very low-probability world of Narnia as the much higher-probability world of next Wednesday's meeting—as free to explore unlikely Middle-earth as the much more predictable park next door.
The point is not that reading fantastic literature or playing fantastic games will make children smarter or more well-adjusted or get better grades in their chemistry classes. Perhaps it's the inevitable constraints of our adult nature that make us think in terms of these practical future questions. But, still, since it's Christmas, we might indulge in a moment or two of sheer childlike pleasure in a beautiful reality. The spirit of possibility and play that leads children to read the Narnia books and watch the Harry Potter movies, and to just imagine, is at the heart of what it is to be human.