How to get all child pages of a parent page in wordpress?

Question

Example:

About
--- Menu 1
--- Menu 2
--- Menu 3
--- Menu 4

if i'm in about page... i have the sub pages. but, if enter to Menu 1 all the pages disappear

What i need is all the time see the parent pages

Currently I have this code

<? if (is_page()) {
    $g_page_id = $wp_query->get_queried_object_id();
    wp_list_pages("depth=4&title_li=&child_of=".$g_page_id."&sort_column=menu_order");
   }
?>

Thanks!

Resolved

i use this and work fine!

<?php
if ( is_page() ) :
    if( $post->post_parent ) :
        $children = wp_list_pages( 'title_li=&child_of='.$post->post_parent.'&echo=0' );
    else;
        $children = wp_list_pages( 'title_li=&child_of='.$post->ID.'&echo=0' );
    endif;
    if ($children) : ?>
        <div class="title">
            <?php
            $parent_title = get_the_title( $post->post_parent );
            echo $parent_title;
            ?>
            <span></span>
        </div>
        <ul>
            <?php echo $children; ?>
        </ul>
    <?php
    endif;
endif;
?>

Answer 1

Here you go. A bit late for the author, but people will come here for an answer still ;-)

<?php 
// determine parent of current page
if ($post->post_parent) {
    $ancestors = get_post_ancestors($post->ID);
    $parent = $ancestors[count($ancestors) - 1];
} else {
    $parent = $post->ID;
}

$children = wp_list_pages("title_li=&child_of=" . $parent . "&echo=0");

if ($children) {
?>

    <ul class="subnav">
        <?php 
            // current child will have class 'current_page_item'
            echo $children; 
        ?>
    </ul>

<?php 
} 
?>