How to show child page without providing ID so it’s dynamic

Question

//The correct code

functions.php

  function get_page_parent_id( $id ) {
$args = array(
  'sort_order' => 'ASC',
  'sort_column' => 'menu_order',
  'child_of' => $id
);
$args = get_pages($args);

if(is_array($pages))
  $pageID = $id;
else {
  $pageID = wp_get_post_parent_id( $id );
}

return $pageID;


}
 ?>

page.php

<?php
          $parentID = get_page_parent_id(get_the_ID());

            $childArgs = array(
            'sort_order' => 'ASC',
            'sort_column' => 'menu_order',
            'child_of' => $parentID
          ); ?>
    <div class="subnav">
      <h3 class="subnav-headline"><a href="https://wordpress.stackexchange.com/over-ons" class="c-dark"><?php echo get_the_title($parentID); ?></a></h3>

        <ul class="menu-items menu-level-1 menu-count-5">
             <?php $pages = get_pages($childArgs);
                foreach($pages as $page ) { ?>
          <li class="menu-item item-number-2 item-id-84286 item-even item-page item-node item-alias-over-ons-geschiedenis-leonidas"><a href="<?php echo get_the_permalink($page);?>"><?php echo $page->post_title;?></a></li>
            <?php   }   ?>
      </ul>
    </div>