Display thumbnail of child, parent and ancestor using featured thumbnails

I think you can use a different approach: write a custom sql query that get the ids of the thumbnails attachment of the children pages, and if found call wp_get_attachment_image_src on this ids to retrieve the urls:

function my_get_thumbnails( $post = NULL, $which="both" ) {
  // first we get the post, if no post is passed we use global post
  if ( empty( $post ) ) global $post;
  if ( is_numeric($post) ) $post = get_post( $post );
  if ( ! $post instanceof WP_Post ) return;
  $children = FALSE;
  $parent = FALSE;
  // if we want children posts thumbnail...
  if ( $which !== 'parent' ) {
    // run a query to get attachment id set as thumbnail in children posts
    global $wpdb;
    $q = "SELECT {$wpdb->postmeta}.meta_value FROM {$wpdb->postmeta}
    JOIN {$wpdb->posts} ON {$wpdb->posts}.ID = {$wpdb->postmeta}.post_id
    WHERE {$wpdb->postmeta}.meta_key = '_thumbnail_id'
    AND {$wpdb->posts}.post_parent = %d";
    $thumb_ids = $wpdb->get_col( $wpdb->prepare( $q, $post->ID ) );
    // if some results, call wp_get_attachment_image_src on all ids to get image urls
    if ( ! empty( $thumb_ids ) ) {
      $children = array_map( function($tid) {
         $img = wp_get_attachment_image_src( $tid, 'large' ); // <-- SET SIZE HERE
         return $img[0];
      }, $thumb_ids );
    }
  }
  // if we want parent post thumbnail...
  if ( $which !== 'children' && $post->post_parent ) {
    $tid = get_post_thumbnail_id( $post->post_parent );
    if ( $tid ) {
      // get the url of parent post thumbnail
      $img = wp_get_attachment_image_src( $tid, 'large' );  // <-- SET SIZE HERE
      $parent = $img[0];
    }
  }
  // if we want only children posts thumbnail return them
  if ( $which === 'children' ) return $children;
  // if we want only parent post thumbnail return it
  if ( $which === 'parent' ) return $parent;
  // if we want bot return an array with both
  return array( 'children' => $children, 'parent' => $parent );
}

After you have entered previous function in functions.php, in your template (probably page.php), use it like so:

<?php
global $post;
if ( is_page() ) {

  $thumbnails = my_get_thumbnails();

  if ( ! empty( $thumbnails['parent'] ) ) {
    $format="<div class="parent-thumb"><img src="https://wordpress.stackexchange.com/questions/139667/%s" alt="" /></div>";
    printf( $format, $thumbnails['parent'] );
  }

  if ( is_array( $thumbnails['children'] ) && ! empty( $thumbnails['children'] ) ) {
    $open = '<div class="child-thumb"><img src="';
    $close="" alt="" /></div>";
    echo $open . implode( $close . $open, $thumbnails['children'] ) . $close;
  }

}
?>

my_get_thumbnails function can be used also to

• get the parent/children post thumbnails also for a post that is not the current, simply passing a post ID or a post object as first argument
• get only the parent post thumbnail passing ‘parent’ as second argument
• get only the children post thumbnails passing ‘children’ as second argument