$child_pages = $wpdb->get_results("SELECT * FROM $wpdb->posts WHERE post_parent = ".$post->ID." AND post_type = 'page' ORDER BY menu_order", 'OBJECT');
if ( $child_pages ) :
foreach ( $child_pages as $pageChild ) :
setup_postdata( $pageChild );
$thumbnail = get_the_post_thumbnail($pageChild->ID, 'thumbnail');
?>
endforeach;
endif;
?>