Get all children page ID’s including parent by title

Question

You have done this the hard way. WP_Query can do most of the work for you.

function get_all_pages($page_title) {
  $page = get_page_by_title($page_title);

  if (empty($page)) return array();

  $children = new WP_Query(
    array(
      'post_type' => 'page',
      'post_parent' => $page->ID,
      'fields' => 'ids'
    )
  );
  array_unshift($children->posts,"{$page->ID}");
  return $children->posts;
}

$allpages = get_all_pages('Sample Page');
var_dump($allpages);

It is late, and I am sleepy, but I think that duplicates your function with the exception that your code, unnecessarily to my mind, produces nested arrays. My code gives you a “flat” array.