Display child pages on a parent and child page using Featured Thumbnails

Use get_ancestors to get the page parent, then get the children of that parent.

$ancestors = array();
$ancestors = get_ancestors($post->ID,'page');
$parent = (!empty($ancestors)) ? array_pop($ancestors) : $post->ID;
if (!empty($parent)) {
  $kids = new WP_Query(
    array(
      'post_parent'=>$parent,
      'post_type' => 'page',
      'ignore_sticky_posts' => true
    )
  );
  if ($kids->have_posts()) {
    while ($kids->have_posts()) {
      $kids->the_post();
      echo '<a href="'.get_permalink().'" title="'.get_the_title().'">'.get_the_post_thumbnail().'</a>';
    }
  }
}