You have to slightly modify your query on your above code as:
$postsForSitemap = get_posts( array(
'numberposts' => -1,
'orderby' => 'rand',
'post_type' => array( 'page' ),
'post_parent' => 0,
'order' => 'DESC'
) );
if you want all page without parent as you have used to get parent pages only.
It will generate a sitemap.xml file and update the links (URLs) with the most recent first (last modified) after you create/update the page.
Hope that helps!!