最后，我自己解决了。以下是解决方案：

在里面functions.php:

function show_all_children($parent_id, $post_id, $current_level)
{   
$top_parents    = array();
$top_parents    = get_post_ancestors($post_id);
$top_parents[]  = $post_id;

$children = get_posts(
    array(
      \'post_type\'       => \'page\'
    , \'posts_per_page\'  => -1
    , \'post_parent\'     => $parent_id
    , \'order_by\'        => \'title\'
    , \'order\'           => \'ASC\'
));

if (empty($children)) return;

echo \'<ul class="children level-\'.$current_level.\'-children">\';

foreach ($children as $child)
{
echo \'<li\';
    if (in_array($child->ID, $top_parents))
    {
    echo \' class="current_page_item"\';
    }
echo \'>\';

echo \'<a href="\'.get_permalink($child->ID).\'">\';
echo apply_filters(\'the_title\', $child->post_title);
echo \'</a>\';

    // now call the same function for child of this child
    if ($child->ID && (in_array($child->ID, $top_parents)))
    {
    show_all_children($child->ID, $post_id, $current_level+1);
    }

echo \'</li>\';
}

echo \'</ul>\';
}

<?php
$parents_ids   = get_post_ancestors($post->ID);
$top_parent_id = (count($parents_ids) > 0) ? $parents_ids[count($parents_ids)-1] : $post->ID;
show_all_children($top_parent_id, $post->ID, 1);
?>