Just had this problem encountered and found out the solution.
If you look at the documentation for $cfs->get() you will see the parameters are:
$cfs->get($field_name, $post_id, $options);
Furthermore if you look at the elements in the $post array you will not find a page_parent, but instead a post_parent
Putting all of this together you should have something like this:
<?php $cfs->get('foo', $post->post_parent); ?>
Hopefully this works for you.
