Define module parent item to contain other module items
What you are trying to achieve is really simple. You need not use any additional code. You just need to follow the correct menu structure. I've posted a sample code below

     function mymodule_menu() {
      $items['admin/config/development/mymodule'] = array(
      //Your code here
      );
     $items['admin/config/development/mymodule/sub-module-1'] = array(
      //Your code here
      );
     $items['admin/config/development/mymodule/sub-module-2'] = array(
      //Your code here
      );
      return $items;

} 


You need to clear the cache. Sometimes you might need to disable and re-enable your modules to register the menu. Now when you click mymodule menu item, a list of all your sub modules will be displayed.