Javascript that will execute on only child pages of a specific parent

You don’t quite cover how far you got and if you are familiar with basic of WP enqueue process.

Focusing on parent part I the basic code would go like this:

add_action( 'wp_enqueue_scripts', function () {

    if ( is_page() && in_array( $book_page_id, get_post_ancestors( get_the_id() ) ) ) {

        // enqueue stuff
    }
} );

In your examples you only have one level and might have worked to just check post_parent of current post, but checking for ancestors is more thorough for more nested cases.