I've always been curious about this since I was a kid, but I never really understood the explanation properly. My son keeps asking me about it now and I'd like to give him a decent answer, not just a "because."
The most important thing to understand is that sunlight actually isn't blue - it's a mix of all colors. What happens is that when light travels through the air, the tiniest particles (like nitrogen and oxygen) bounce blue light around much more easily than red or yellow light. This phenomenon is called Rayleigh scattering, and here's how it works: blue light has a shorter wavelength, so it crashes into those microscopic particles and bounces all over the sky, while red light, which has a longer wavelength, passes straight through without bouncing much.
To explain it to your son in a way he'll get, you can use a metaphor: imagine blue light like a ping pong ball and red light like a soccer ball. If you throw them at a net, the small ball bounces everywhere and you see it from lots of angles, while the big one passes through more easily. That's why when you look at the sky in any direction you mainly see blue - it's like all that light is bouncing back toward your eyes.
Another cool detail is that at sunset the sky turns red-orange, precisely because light has to travel through way more atmosphere to reach us. At that point all the blue light has been scattered away along the way, and only the red light is left. It might seem simple and boring, but the fact that the color changes depending on how far the light travels - that makes it pretty fascinating even for a kid.
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