Add button to menu item at the same row

Please try something like this.

    function add_button( $output, $item, $depth, $args ){
          if (in_array("menu-item-has-children", $item->classes)) {
              $output="<button type="button" class="btn btn-circle">
<i class="fa fa-chevron-circle-down"></i> 
</button>".$output;
          }
          return $output;
      } 
      add_filter( 'walker_nav_menu_start_el', 'add_button',10,4);