What child hasn’t wondered and asked this question. Answering it isn’t all that hard but it helps if you have a good enough understanding of visible light and wavelengths to explain the concept. Lacking that, it’s just important to understand that visible light (there are more spectrums of light than just those you see) comes in colors as is visible in a rainbow. Each color has a different wavelength. The reason our sky appears blue is that the blue wavelength of light doesn’t pass through our atmosphere as efficiently as other wavelengths. As light enters our atmosphere, the molecules of gas absorb and release it to varying degrees. Red and orange, for example, pass through to the planet more readily than blue. Once absorbed, the light is eventually released in the same color but potentially in different directions, scattering the light from its original source.
Since blue light is more commonly absorbed and then released, you can imagine it being scattered all over the sky more so than other colors which, in turn, means the sky appears blue from anywhere on Earth. You can read a more in-depth explanation about light over at Science Made Simple.