add_filter( 'walker_nav_menu_start_el', 'generate_tertiary_nav_dropdown', 10, 4 );
function generate_tertiary_nav_dropdown( $item_output, $item, $depth, $args )
{
	// If we're working with the primary or secondary theme locations
	if ( 'tertiary' == $args->theme_location ) {
		// If a dropdown menu is detected
		$dropdown = ( in_array( 'menu-item-has-children', $item->classes ) || in_array( 'page_item_has_children', $item->classes ) ) ? true : false;
		if ( $dropdown ) :
			// Add our arrow icon
			$item_output = str_replace( $args->link_after . '</a>', $args->link_after . '<span role="button" class="dropdown-menu-toggle" aria-expanded="false"></span></a>', $item_output );
		endif;
	}

	// Return the output
	return $item_output;
}