In the 1870s, the British physicist Lord Rayleigh began investigating why the daytime sky appears bright blue. In 1899 he published a paper on the origin of the blue of the sky. His work showed that the tiny gas molecules in the air scatter sunlight.

Rayleigh established a fundamental mathematical relationship: the intensity of light scattering is inversely proportional to the fourth power of its wavelength. Because of this steep mathematical ratio, short wavelengths of light such as violet and blue (roughly 400 to 490 nanometres) are scattered far more strongly by air molecules than longer wavelengths like red light (around 700 nanometres).

At sunset, sunlight must travel through a much thicker layer of atmosphere to reach an observer, dispersing most of the violet and blue light out of the direct beam and allowing warmer red and orange hues to dominate. Although violet wavelengths scatter even more efficiently than blue ones, our daytime sky appears blue because the Sun emits less violet radiation, the upper atmosphere absorbs some of it, and human eyes are simply less sensitive to violet light.

I think it is remarkable that a simple question like this leads to real physics. It seems to me that it is worth asking children to look for the evidence themselves.