Why is the voice of men, women and children different?

The voice of a human depends upon length of  their vocal cords.

The vocal cord of a man is about 20 mm long.

The vocal cord of a woman is about 5 mm long.

The vocal cord of a child is very small.

Frequency of the voice depends upon the length of the vocal cord.

Shorter the vocal cord, more is the frequency and hence more shrill is the voice.

Due to the difference in length of vocal cords, the sounds produced by the three is different.