Why is the sky blue?

The sky is blue because air scatters the blue part of sunlight more than the red or yellow parts. Sunlight contains many colors together. When it passes through the atmosphere, tiny gas molecules redirect the shorter blue wavelengths, so blue reaches your eyes from across the sky.

Violet light gets redirected too, even more strongly, but we do not see much purple overhead. The sun gives us less violet light, and human eyes notice blue more easily. So the color your child points to is a partnership between sunlight, air, and eyes.