From your question I understand that inside the top level page you want to indicate which child page to include. The obvious way to do this is using a shortcode. So in your functions.php you would have something like this:
add_shortcode( 'insert_child', 'wpse240522_insert_child' );
In the content of your top level page you would have [insert_child 23], where 23 is the ID of the child page.
Now you must define the function that evaluates the shortcode, first getting the child page and then looping through all the grandchild pages:
function wpse240522_insert_child ($child_id) {
$child_page = get_post ($child_id);
// do stuff with the child page
$grandchild_pages = new WP_Query (array ('post_parent' => $child_id));
foreach ($grandchild_pages as $grandchild_page) {
// do stuff with grandchild page
}
}