class: admin:ingredient extends :x:primitive {
  protected function stringify() {
    foreach ($this->getChildren() as $child) {
      $output .= :x:base::renderChild($child);
    }
    $output .= '<div> flour </div>';
    return $output;
  }
}

$elem = <admin:ingredient> <div>sugar</div> </admin:ingredient>;
echo $elem;