page.php

<?php 
    get_header();
    while(have_posts()){
        the_post();?>
        <div class="page-banner">
      <div class="page-banner__bg-image" style="background-image: url(<?php echo get_theme_file_uri('images/ocean.jpg')?>)"></div>
      <div class="page-banner__content container container--narrow">
        <h1 class="page-banner__title"><?php the_title();?></h1>
        <div class="page-banner__intro">
          <p>Learn how the school of your dreams got started.</p>
        </div>
      </div>
    </div>

    <div class="container container--narrow page-section">
        <?php
             $theParent = wp_get_post_parent_id(get_the_ID());
             if($theParent){?> <!-- $theParent holds the parent id -->
                <div class="metabox metabox--position-up metabox--with-home-link">
                <p>
                  <a class="metabox__blog-home-link" href="<?php echo get_permalink($theParent);?>"><i class="fa fa-home" aria-hidden="true"></i> Back to <?php echo get_the_title($theParent);?></a> <span class="metabox__main"><?php the_title();?></span>
                </p>
              </div>
            <?php }
        ?>
      <?php
      $testArray = get_pages(array( # child of the current post/page
          'child_of' => get_the_ID()
      ));
      if($theParent or $testArray){ ?><!-- if the current post has a parent or if it's child post-->
      <div class="page-links">
        <h2 class="page-links__title"><a href="<?php echo get_permalink($theParent); ?>"><?php echo get_the_title($theParent);?></a></h2>
        <ul class="min-list">
          <?php
           if($theParent){ # for the child post
               $findChildrenOf = $theParent;
           } else { # for the parent post
            $findChildrenOf = get_the_ID();
           }
            wp_list_pages(array(
                'title_li' => NULL,
                'child_of' => $findChildrenOf,
                'sort_column' => 'menu_order'
            ));
          ?>
        </ul>
      </div>
      <?php }?>

      <div class="generic-content">
       <?php the_content();?>
      </div>
    </div>
        <?php
    }
    get_footer();
?>