menus with sub and sub sub menus

Hi, i am trying to create a menu of this sort. https://www.gatewayhouse.in/
and the code i am using is

class reverie_custom_walker extends Walker_Nav_Menu {

        // add classes to ul sub-menus
        function start_lvl( &$output, $depth ) {
            // depth dependent classes
            $indent = ( $depth > 0  ? str_repeat( "\t", $depth ) : '' ); // code indent
            $display_depth = ( $depth + 1); // because it counts the first submenu as 0
            $classes = array(
            'sub-menu',
            ( $display_depth % 2  ? 'menu-odd' : 'menu-even' ),
            ( $display_depth >=2 ? 'sub-sub-menu' : '' ),
            'menu-depth-' . $display_depth
            );
            $class_names = implode( ' ', $classes );

            // build html
            if($depth == 0){
                $output .= "\n" . $indent . '<ul class="' . $class_names . '">' . "\n";
            }
        }

        // add main/sub classes to li's and links
        function start_el( &$output, $item, $depth, $args ) {
            global $wp_query;
            $indent = ( $depth > 0 ? str_repeat( "\t", $depth ) : '' ); // code indent

            // depth dependent classes
            $depth_classes = array(
            ( $depth == 0 ? 'main-menu-item' : 'sub-menu-item' ),
            ( $depth >=2 ? 'sub-sub-menu-item' : '' ),
            ( $depth % 2 ? 'menu-item-odd' : 'menu-item-even' ),
            'menu-item-depth-' . $depth
            );
            $depth_class_names = esc_attr( implode( ' ', $depth_classes ) );

            // passed classes
            $classes = empty( $item->classes ) ? array() : (array) $item->classes;
            $class_names = esc_attr( implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) ) );

            // build html
            echo $depth;
            if($depth == 0){
                $output .= $indent . '<li id="nav-menu-item-'. $item->ID . '" class="' . $depth_class_names . ' ' . $class_names . '">';
            }
            elseif($depth == 1){
                $output .= $indent . '<span id="nav-menu-item-'. $item->ID . '" class="' . $depth_class_names . ' ' . $class_names . '"> ';
            }
            elseif($depth == 2){
                $output .= '<div class="row">';
                $output .= $indent . '<div id="nav-menu-item-'. $item->ID . '" class="six columns' . $depth_class_names . ' ' . $class_names . '">';

            }

            // link attributes
            $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
            $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
            $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
            $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
            $attributes .= ' class="menu-link ' . ( $depth > 0 ? 'sub-menu-link' : 'main-menu-link' ) . '"';

            //echo $item->url;

            // print_r(  $args);
            $item_output = sprintf( '%1$s<a%2$s>%3$s%4$s%5$s</a>%6$s',
            $args->before,
            $attributes,
            $args->link_before,
            apply_filters( 'the_title', $item->title, $item->ID ),
            $args->link_after,
            $args->after
            );

            // build html
            $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
        }
        function end_el( &$output, $item, $depth = 0, $args = array() ) {

            if($depth == 0){
                $output .= "</li>\n";
            }
            elseif($depth == 1){
                $output .= "</span>\n";
            }
            elseif($depth == 2){
                $output .= "</div> </div>\n";

            }

            //$output .= "</li>\n";
        }
        function end_lvl( &$output, $depth = 0, $args = array() ) {
            $indent = str_repeat("\t", $depth);
            if($depth == 0){
                $output .= "$indent</ul>\n";
            }
        }
    }

I am looking for a function which will give me child menus if i sent the parent ID to it.

Thanks