How do I change a parent theme’s function through the child theme?

The parent theme’s functions.php will load automatically. You don’t need to “read” it, and you don’t want to “hack” it either manually, which will be overwritten, or programatically, which is very resource intensive.

Your theme is enqueueing the script on the wp_head action. You just need to remove that callback and add a slightly modified one. The trick is that your child theme’s functions.php loads after your parent functions.php so you need to get the timing right.

function dequeue_dt_specific_enqueues() {
  remove_action('wp_head', 'dt_specific_enqueues');
}
add_action('after_setup_theme', 'dequeue_dt_specific_enqueues');

Then add back your own version:

function altered_dt_specific_enqueues() {
  if (is_single()) {
    wp_enqueue_script( 'comment-reply' );
  }
}
add_action('wp_head', 'altered_dt_specific_enqueues');