How to automatically create grandchild pages to subpages - Wordpress?

I have a custom post type 'Departments' in Wordpress. When I create a page in this CPT, it automatically creates 3 subpages. That works fine!

But now I want two grandchildren pages to be created automatically and their parent must be Subpage2.

Right now I have it working like this:

Department1
-Subpage1
-Subpage2
-Subpage3

Department2
-Subpage1
-Subpage2
-Subpage3
-Subpage4

And I need it to be like this:

Department1
-Subpage1
-Subpage2
  --GrandchildrenPage1  <--
  --GrandchildrenPage2  <--
-Subpage3

Department2
-Subpage1
-Subpage2
  --GrandchildrenPage1  <--
  --GrandchildrenPage2  <--
-Subpage3

I currently have this code that works fine to create the 3 subpages:

// Add 3 Children CPT
function add_children_custom_post_type( $post_id ) {  
if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )
    return;

$post_template = esc_html( get_page_template_slug( $post->ID ) );
if ( !wp_is_post_revision( $post_id ) && 'department' == get_post_type( $post_id ) && 'publish' == get_post_status( $post_id ) && $post_template === '') {
    $show = get_post( $post_id );

    if( 0 == $show->post_parent && $show->post_excerpt == "" ) {
        $children =& get_children(
            array(
                'post_parent' => $post_id,
                'post_type' => 'department',
                'post_status' => array('publish', 'pending', 'draft', 'auto-draft', 'future', 'private', 'inherit', 'trash')
            )
        );
        if( empty( $children ) ){
            $posts = "";
            $posts  = ['Subpage1' => 'template-subpage1.php', 'Subpage2' => 'template-subpage2.php', 'Subpage3' => 'template-subpage3.php'];
            $order = 1;
            foreach ($posts as $title => $template) {
                $child = array(
                    'post_type' => 'department',
                    'post_title' => $title,
                    'post_status' => 'draft',
                    'post_parent' => $post_id,
                    'post_author' => get_post_field('post_author', $post_id),
                    'menu_order' => $order,
                    'page_template' => $template
                );
                wp_insert_post( $child );
                $order++;
            }
        }
     }
  }
}
add_action( 'save_post', 'add_children_custom_post_type' );