get a list of sub pages using wp_list_pages(); function for more information visit codex
$args = array('child_of' => 27);
wp_list_pages( $args );
this will print a list of sub pages (child) of page 27, sort alphabetically order, also you can get a list of sub pages by doing this another way like below.
wp_list_pages( 'child_of=27' );