As a child your Larynx and Vocal cords are smaller in size.
The length of the trachea is also shorter in children than in adults which is another cause of the high pitched voice.
Hormones also factor into the pitch of the voice and since children have less hormones in their bodies that contributes to the child's voice being higher pitched as well.
Testosterone causes the muscles in the trachea (what we colloquially refer to as the voice box) to harden, and this also had an impact on the resonances within the trachea.
As the child grows the voice box, trachea and Larynx also grows with the child and as they grow the voice gets lower in pitch and better sounding.
As a child reaches puberty then the child's voice begins to crack which can be embarrassing but everyone has gone through that experience.