My question is how can I automatically get the parent post content on
every child
Your code should work just like it is: if you have a parent and you add any number of children to this parent (wpse121567_get_cpt_hierarchy return ‘debate-child’ for them), your code will show the content of parent post.
and if I add more content from the dashboard to be added after the
content from the parent element?
for this, where you have:
global $post;
$parent = get_post( $post->post_parent );
echo '<h1>' . $parent->post_title . '</h1>';
echo '<div>' . apply_filters( 'the_content', $parent->post_content ) . '</div>';
replace with
the_post();
$parent = get_post( $post->post_parent );
echo '<h1>' . $parent->post_title . '</h1>';
$content = apply_filters( 'the_content', $parent->post_content ) . get_the_content();
echo '<div>' . $content . '</div>';