gistfile1.phtml

/*FUNCTION CODE HERE */
function get_top_parent_page_id() {

    global $post;

    // Check if page is a child page (any level)
    if ($post->ancestors) {

        // Grab the ID of top-level page from the tree
        return end($post->ancestors);

    } else {

        // Page is the top level, so use it's own id
        return $post->ID;

    }

}

function is_subpage() {
    global $post; // load details about this page

    if ( is_page() && $post->post_parent ) { // test to see if the page has a parent
        return $post->post_parent; // return the ID of the parent post

    } else { // there is no parent so ...
        return false; // ... the answer to the question is false
    }
}

/*TEMPLATE CODE HERE */

<aside id="sidebar" class="col-right">
  		<div class="child_page_list">
				<ul>
				<?php
					if (is_subpage()):
						wp_list_pages('title_li=&child_of='.get_top_parent_page_id()); 
					else:
						$children = get_children($post->ID);
						if (empty($children)):
							wp_list_pages('title_li=&include='.$post->ID);
						else:
							wp_list_pages('title_li=&child_of='.get_top_parent_page_id()); 
						endif;
					endif;
				?>
				</ul>
			</div>
		</aside>