@brett I did something similar for an ACF block where I wanted to automatically show child pages, I set a var using $page_id = get_queried_object_id(); and used that var to set the post parent:
$page_id = get_queried_object_id();
$args = array(
'post_type' => 'page',
'posts_per_page' => -1,
'post_parent' => $page_id,
'order' => 'ASC',
'orderby' => 'menu_order'
);
$parent = new WP_Query( $args );
if ( $parent->have_posts() ) : ?>
This worked on the front end. Unfortunately for me this caused an issue in the WordPress admin – in WordPress admin screen all top level pages were showing. The slowed down my editing experience immensley so I instead opted to set the query using a tag instead of post parent.
