Get Child Page IDs by Parent ID

This should work. I’m not entirely sure what is wrong with your code (it’s getting late) but the below code works on my end. Obviously I replaced $order->get_id() with a known ID and post_type was set to page.

<?php 
    $parentid = $order->get_id();

    $child = new WP_Query( array('post_parent' => $parentid, 'post_type' => 'shop_subscription') );

    if ($child->have_posts()) : while ($child->have_posts()) : $child->the_post();
        $childid = get_the_ID();
    endwhile;
        else:
            $childid = "not set";
    endif;

    wp_reset_query();

?>