Joomla在模块中获取子类别的文章内容

How can I include article data from database of a subcategory? I have successfully got the subcategory using JCategories API helper, however I have not yet found a way to get the article in the subcategory below is my module helper file:

defined('_JEXEC') or die;
jimport('joomla.application.categories');
class Modtopviewcourses extends JCategories
{ 
     public static function getdata($catid)
    {
         $categories = JCategories::getInstance('Content');
          $cat = $categories->get($catid);
          $children = $cat->getChildren();
          $res=array();
          $res['articles']=$children;
          return $res;
     }
}