<?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;

?>