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function get_top_parent_page_id() {
 
    global $post;
 
    $ancestors = $post->ancestors;
 
    // Check if page is a child page (any level)
    if ($ancestors) {
 
        //  Grab the ID of top-level page from the tree
        return end($ancestors);
 
    } else {
 
        // Page is the top level, so use  it's own id
        return $post->ID;
 
    }
 
}

<ul>
<?php wp_list_pages("child_of=$parent_page&title_li="); ?>
</ul>