List pages including both parent and child pages

I don’t think there is a way to express that precisely in its arguments.

You could work around that by building array of page IDs which includes parent and children set as desired.

$pages = get_posts( [
    'post_parent' => $page_id,
    'post_type'   => 'page',
    'fields'      => 'ids',
] );

$pages[] = $page_id;

wp_list_pages( [
    'include' => $pages,
] );