used below jquery function to add functionality to slide toggle the submenu of the menu items which has got children for the mobile viewport.
$('.submenu-open').click(function(e) {
$('.menu-item-has-children li').slideToggle('slow');
});
But, it opens all the menu items with children at the same time (on mobile viewport), But it should only open the submenu of that menu item.
here is the link