How to restrict page access to logged-in users?

by

How about this?

function my_force_login() {
  global $post;

  if (!is_single()) return;

  $restricted_parent_id = 1 // Whatever id you are targeting.

  if ($post->post_parent == $restricted_parent_id && !is_user_logged_in()) {
    auth_redirect();
  }
}

More info on post_parent and other conditional tags here:
https://codex.wordpress.org/Conditional_Tags