Kids have lots of questions about the nature of the world, and this is good. However, I think our answers to these questions often miss the mark. Consider that quintessential chestnut, "Why is the sky blue?" Google for an answer and you will find physical explanations involving atmospheric molecules, light scattering, and wavelengths of light. Let's focus on the color aspect, because here the explanation given to children and adults is about the same. The color model is that white light, such as that given by the Sun, is actually a bundle of waves, where each color is a different wavelength.
I remember reading such an explanation in one of W's "science for kids" books and feeling uneasy. I did not like the matter-of-fact approach and authoritative tone, as if the question and answer belonged to the same category of question as "What is the state capital of Massachusetts?". It seemed to me that something important was left out of the answer.
One source of my uneasiness has to do with how much explanatory power we can ascribe to physical models at all and how we interpret the reality of those models. I could write at great length here, but I'll admit this is mostly metaphysical quibbling. Given the success of physics in getting men to the Moon and back, I am willing to put aside those questions... at least until the boys are a little older.
But even if we take the physics at face value, I still have uneasiness. Doess the "color is really just a wavelength of electromagentic radiation" answer really do full justice to the phenomenon of color?
Recently I came across a thought experiment that articulates my unease: Mary's Room. The idea is to conjure someone who has never seen color because she has been captive in a colorless room her whole life. Now give her perfect knowledge of the physics involving color, atmosphere, and anything else you like. The question is this: when she is released from the room and looks up at the sky for the first time, will she learn anything new about color?
Of course she will. She will learn what color looks like. And here we come to what it is that physics necessarily leaves out: experience. Because of this, we should not pretend that physical science provides complete explanations for the nature of things.
Let's get back to the child asking why the sky is blue. Maybe "color is a wavelength" needs to be part of our answer. But maybe part of our answer also needs to be "color is a particular experience". Which is a small child really reaching for: the mechanics of light waves, or wonderment at why an experience is just this experience and not that experience.
As a philosopher might say, perhaps whatever answer we give should end with a semi-colon instead of a period.