WordPress - 将NavWalker设置为仅在单击图像时显示子站点

Question

I need some adivce on how to set up my NavWalker for Wordpress. This is my Walker (copied, working so far):

> class CSS_Menu_Walker extends Walker { 

    var $db_fields = array('parent' => 'menu_item_parent', 'id' => 'db_id');

    function start_lvl(&$output, $depth = 0, $args = array()) {
        $indent = str_repeat("\t", $depth);
        $output .= "
$indent<ul class=nav>
";
    }

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

    function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0) {

        global $wp_query;
        $indent = ($depth) ? str_repeat("\t", $depth) : '';
        $class_names = $value = '';
        $classes = empty($item->classes) ? array() : (array) $item->classes;

        /* Add active class */
        if (in_array('current-menu-item', $classes)) {
            $classes[] = 'active';
            unset($classes['current-menu-item']);
        }

        /* Check for children */
        $children = get_posts(array('post_type' => 'nav_menu_item', 'nopaging' => true, 'numberposts' => 1, 'meta_key' => '_menu_item_menu_item_parent', 'meta_value' => $item->ID));
        if (!empty($children)) {
            $classes[] = 'has-sub';
        }

        $class_names = join(' ', apply_filters('nav_menu_css_class', array_filter($classes), $item, $args));
        $class_names = $class_names ? ' class="' . esc_attr($class_names) . '"' : '';

        $id = apply_filters('nav_menu_item_id', 'menu-item-'. $item->ID,
        $item, $args);
        $id = $id ? ' id="' . esc_attr($id) . '"' : '';

        $output .= $indent . '<div><li' . $id . $value . $class_names
.'>';

    $attributes  = ! empty($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       ) .'"' : '';

        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'><span>';
        $item_output .= $args->link_before . apply_filters('the_title',
    $item->title, $item->ID) . $args->link_after;
        $item_output .= '</span></a>';
        $item_output .= $args->after;

        $output .= apply_filters('walker_nav_menu_start_el', $item_output, $item, $depth, $args);
    }

    function end_el(&$output, $item, $depth = 0, $args = array()) {
        $output .= "</li></div>
";
    }

}

My task now is to create a clickable image before each List-Item which has children. After that click the child-nodes should be expanding beneath the List-Item i clicked the nearby picture.

I've been able to get the image right before every List-Item and Sub-Item with:

> wp_nav_menu array (
'theme_location' => 'primary',
'container_id' => 'cssmenu',
'link_before' => 'some HTML with img',
'walker' => new CSS_Menu_Walker () 
) );

Right now im stuck by identifying the List-Items with child-nodes. And even if i manage to get them identified somehow in my walker, how can i place an image in front of only those with childs?