code: public function getDirectChildren(array $filter = [])
      {
          return static::query()
              ->filter($filter)
              ->filter(['SECTION_ID' => $this->id])
              ->select('ID')
              ->getList()
              ->transform(function ($section) {
                  return (int) $section['ID'];
              })
              ->all();
      }
docs: Get IDs of direct children of the section.
  Additional filter can be specified.
  
  @param array $filter
  
  @return array