public function getChildren()
 {
     $children = array();
     // static directories
     foreach (static::$siteDirectories as $name => $class) {
         $instance = $this->getChild($name);
         if ($instance->getName()) {
             $children[] = $instance;
         }
     }
     // merge with root virtual collections
     return array_merge($children, Collection::getAllRootCollections());
 }