Creates a mobile responsive menu in a Genesis child theme with support for collapsible sub menus.

responsive-menu-css.css

/* Responsive Navigation
---------------------------------------------------------------------------------------------------- */

/* Standard Navigation
--------------------------------------------- */

nav {
	clear: both;
}

/* Navigation toggles
--------------------------------------------- */

.sub-menu-toggle,
.menu-toggle {
	display: none;
	visibility: hidden;
}

/* Navigation toggles - Ensure Menu Displays when Scaled Up
--------------------------------------------- */

@media only screen and (min-width: 768px) {

	nav {
		display: block !important;
	}
}

/* Navigation toggles - Mobile (Change max width as you see fit)
--------------------------------------------- */

@media only screen and (max-width: 767px) {

	.menu-toggle,
	.sub-menu-toggle {
		display: block;
		font-size: 20px;
		font-size: 2rem;
		font-weight: 700;
		margin: 0 auto;
		overflow: hidden;
		padding: 20px;
		padding: 2rem;
		text-align: center;
		visibility: visible;
	}

	button.menu-toggle,
	button.sub-menu-toggle {
		background-color: transparent;
		color: #999;
	}

	.sub-menu-toggle {
		padding: 18px;
		padding: 1.8rem;
		position: absolute;
		right: 0;
		top: 0;
	}

	.menu-toggle:before {
		content: "\2261";
	}

	.menu-toggle.activated:before {
		content: "\2191";
	}

	.sub-menu-toggle:before {
		content: "+";
	}

	.sub-menu-toggle.activated:before {
		content: "-";
	}

	nav {
		display: none;
		position: relative;
	}

	.genesis-nav-menu .menu-item {
		background-color: #f5f5f5;
		display: block;
		position: relative;
		text-align: left;
	}

	.genesis-nav-menu .menu-item:hover {
		position: relative;
	}

	.genesis-nav-menu .sub-menu {
		clear: both;
		display: none;
		opacity: 1;
		position: static;
		width: 100%;
	}

	.genesis-nav-menu .sub-menu a {
		border-left: 0;
		position: relative;
		width: auto;
	}

	.genesis-nav-menu .sub-menu .sub-menu {
		margin: 0;
	}

	.genesis-nav-menu .sub-menu .sub-menu a {
		background-color: #f5f5f5;
		padding-left: 30px;
	}

	.genesis-nav-menu .sub-menu .sub-menu .sub-menu a {
		background-color: #fff;
		padding-left: 40px;
	}

	.nav-primary a:hover,
	.nav-primary .current-menu-item > a {
		color: #333;
	}

}

responsive-menu-js.js

( function( window, $, undefined ) {
	'use strict';

	$( 'nav' ).before( '<button class="menu-toggle" role="button" aria-pressed="false"></button>' ); // Add toggles to menus
	$( 'nav .sub-menu' ).before( '<button class="sub-menu-toggle" role="button" aria-pressed="false"></button>' ); // Add toggles to sub menus

	// Show/hide the navigation
	$( '.menu-toggle, .sub-menu-toggle' ).on( 'click', function() {
		var $this = $( this );
		$this.attr( 'aria-pressed', function( index, value ) {
			return 'false' === value ? 'true' : 'false';
		});

		$this.toggleClass( 'activated' );
		$this.next( 'nav, .sub-menu' ).slideToggle( 'fast' );

	});

})( this, jQuery );

responsive-menu-script-enqueue.php

<?php

// Don't add the opening <?php tag

// Register responsive menu script
add_action( 'wp_enqueue_scripts', 'prefix_enqueue_scripts' );
/**
 * Enqueue responsive javascript
 * @author Ozzy Rodriguez
 * @todo Change 'prefix' to your theme's prefix
 */
function prefix_enqueue_scripts() {

	wp_enqueue_script( 'prefix-responsive-menu', get_stylesheet_directory_uri() . '/lib/js/responsive-menu.js', array( 'jquery' ), '1.0.0', true ); // Change 'prefix' to your theme's prefix

}