This question is often asked by children, but the explanation is physically. The sunlight coming from the space collides with the atmosphere and the following happens: Light is an electromagnetic wave. If light collides with the molecules of the air, it is scattered. The wavelength of the light, that is visible for human beings, ranges from about 400 nm to 800 nn (nm = nanometer = 1⋅10-9 meters = 1 billionth of 1 meter). The size of the air molecules is many times smaller than the wavelength of visible light. In this case Rayleigh scattering occurs at the molecules. The decisive factor: in case of Rayleigh scattering the intensity of the scattering is proportional to λ-4 (= 1 ⁄ λ4), where λ is the wavelength of the light. Broadly speaken this means, that the scattering is highly dependent on the wavelength, thus on the color of the light.
Blue light is at the short-wave section of the visible light spectrum and red is at the long-wave one. Now we assume a realistic wavelength of 400 nm for blue light and 800 nm for red light and calculate λ4 of both wavelengths. The difference of the two results is a factor of 16. Referred to Rayleigh scattering this means, that the scattering of the blue light is 16 times stronger compared to red light! If we observe scattered light - and this is exactly that light, which comes not directly from the sun - the blue fraction is much stronger compared to the other colors. And that is the reason why the sky is blue!
Remark: If the scattering particles are much larger, for example cloud droplets, the Rayleigh scattering no longer takes place. Is the size of the light in the order the particles, Mie scattering is valid. Now the intensity is only a bit dependent on the wavelength, so that all colors get more or less the same scattering. And what is the result, if all colors are present: white. Therefore clouds appear to be white - and now tell all these things your nine-year-old child - well.