<?php

$defaults = array(
'depth'         => 1,
'title_li'      => '',
'echo'          => 1,
'exclude'       => 6,
'sort_column'   => 'menu_order, post_title',
'sort_order'    => 'asc',
'link_before'   => '',
'link_after'    => '');

// get all the top level pages
$pages = get_pages($defaults);

// loop around all the top level pages
foreach($pages as $page) {

    // does this page have children?
    $child_pages = get_pages('child_of='.$page->ID.'&sort_column=post_date&sort_order=desc');

    $count = 0;

    $page_multi_ar = array();

    // current page has children...
    if(count($child_pages)>0) {
        // get the child...
        foreach($child_pages as $child_page)
        {
            // store the pageid,childpageid => childpage object
            $page_multi_ar[$page->ID][$child_page->ID] = $child_page;
        }
    // top level page
    } else {
        // store the pageid,0 => page object
        $page_multi_ar[$page->ID][0] = $page;
    }
}

/*
// so you end up with

array[1][0] // top level page
array[2][3] // child of two
array[2][4] // child of two
array[2][5] // child of two
array[6][0] // top level page
array[7][8] // child of eight
*/

//then foreach around the whole array..

$page_multi_ar[1][0] ="top";// top level page
$page_multi_ar[2][3] ="child";// child of two
$page_multi_ar[2][4] ="child";// child of two
$page_multi_ar[2][5] ="child";// child of two
$page_multi_ar[6][0] ="top";// top level page
$page_multi_ar[7][8] ="top";// child of eight

echo'<ul>';

//print_r($arry_);
foreach($page_multi_ar as $parent_id => $inner_ar) {

    $count = 0;

    foreach($inner_ar as $id => $data) {

        if($id!=0) {

            if($count==0) {

                echo '<li>parent_id: '.$parent_id.'</li>
                      <ul><li>';

            } else {

                echo '<li>';

            }

            echo'child_id '.$id;

            if($count==sizeof($inner_ar)-1) { echo '</li></ul>'; } else { '</li>'; }
            $count++;
        // 0 = parent page
        } else {
            echo'<li>parent_id: '.$parent_id.'</li>';
        }
    }
}
echo'</ul>';
?>