loadBookChildren example

  /** * {@inheritdoc} */
  public function deleteFromBook($nid) {
    $original = $this->loadBookLink($nid, FALSE);
    $this->bookOutlineStorage->delete($nid);

    if ($nid == $original['bid']) {
      // Handle deletion of a top-level post.       $result = $this->bookOutlineStorage->loadBookChildren($nid);
      $children = $this->entityTypeManager->getStorage('node')->loadMultiple(array_keys($result));
      foreach ($children as $child) {
        $child->book['bid'] = $child->id();
        $this->updateOutline($child);
      }
    }
    $this->updateOriginalParent($original);
    $this->books = NULL;
    Cache::invalidateTags(['bid:' . $original['bid']]);
  }