code: public function addChild(ModelInterface $child, $captureTo = null, $append = null)
      {
          parent::addChild($child, $captureTo, $append);
  
          if ($child instanceof ThemeDefaultInterface) {
              $child->parent = $this;
          }
  
          return $this;
      }
docs: Add a child model
  
  @param  ModelInterface $child
  @param  null|string    $captureTo Optional; if specified, the "capture to" value to set on the child
  @param  null|bool      $append    Optional; if specified, append to child  with the same capture
  
  @return ViewModel