eilotshai

Ok i found the solution
For those who would like to have 2 differents menu 1 for Mobile device and One for Desktop using the twenty fourteen child-theme here is the solution

1. in your function.php

function register_my_menu() {
  register_nav_menu('mobile-menu',__( 'Menu Mobile' ));
}
add_action( 'init', 'register_my_menu' );

2. Go in your admin panel and create a new menu

3. in your header.php

<?php if ( wp_is_mobile() ) {
				  		echo wp_nav_menu( array( 'theme_location' => 'mobile-menu', 'menu_class' => 'nav-menu' ) );
					}
					else {
						echo wp_nav_menu( array( 'theme_location' => 'primary', 'menu_class' => 'nav-menu' ) );
					}?>

And it should be done!