<?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($children)) return;
echo '
 <a class="top_parent" href="'.get_permalink($parent_id).'" title="'.get_the_title($parent_id).'">'.get_the_title($parent_id).'</a>';


echo '<ul class="side-menu 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>';
}