You know why children are unable to sit still and listen to music?
When you find yourself sitting still and listening to music as an adult, you do so because you become lost in your thoughts. The music stirs emotions, memories, and thoughts you have developed over your years of life. The music transports you through time.
As a child, one does not have the depth of experience to trigger the same emotions or thoughts when listening to music. Instead, kids focus on the present. They hear how the music sounds right this moment. These thoughts differ from what goes through the head of an adult, hence the disinterest in music.
Only once a child reaches a certain depth of experience are they capable of just sitting and listening to music while doing nothing else.