<?php 
// Main loop

if ( have_posts() ) while ( have_posts() ) : the_post();

	// Parent (current) page loop

endwhile;

// Child pages loop

$childargs = array(
	'post_parent' => $post->ID,
	'post_type' => 'page',
	'posts_per_page' => '-1',
	'orderby' => 'menu_order',
	'order' => 'ASC' 
);

$child = new WP_Query($customargs);

if ($child->have_posts()) : while ($child->have_posts()) : $child->the_post();

	/// Loop through child pages of current page

endwhile; endif;

?>