// in your functions.php file add this function

function neighbors($link) {
    global $post;
    $neighbors = get_pages('child_of='.$post->post_parent.'&parent='.$post->post_parent);
    foreach ($neighbors as $key=>$neighbor){
        if ($post->ID == $neighbor->ID){
            $ID = $key;
        }
    }
    $closest = array('before'=>get_permalink($neighbors[$ID-1]->ID),'after'=>get_permalink($neighbors[$ID+1]->ID));

    if ($link == 'before' || $link == 'after') { echo $closest[$link]; } else { return $closest; }
}

//in your template do this

<?php neighbors('before'); ?>
<?php neighbors('after'); ?>