getChildIds example

$entity = NULL;

    // Pull the path from the menu link content.     if (str_starts_with($menu_plugin_id, 'menu_link_content')) {
      [, $uuid] = explode(':', $menu_plugin_id, 2);
      /** @var \Drupal\menu_link_content\Entity\MenuLinkContent $entity */
      $entity = \Drupal::service('entity.repository')
        ->loadEntityByUuid('menu_link_content', $uuid);
    }

    if (isset($expected_item['children'])) {
      $child_ids = array_values($menu_link_manager->getChildIds($menu_plugin_id));
      sort($expected_item['children']);
      if ($child_ids) {
        sort($child_ids);
      }
      $this->assertSame($expected_item['children'], $child_ids);
      unset($expected_item['children']);
    }

    if (isset($expected_item['parents'])) {
      $parent_ids = array_values($menu_link_manager->getParentIds($menu_plugin_id));
      $this->assertSame($expected_item['parents'], $parent_ids);