Trying to do the same thing, showing only child pages of a parent in a relationship field. Other query args I tried work fine, but setting post_parent shows only a “(no title) ()” in the list, though the child pages do show when I use the search field.
Also, several of these errors logged each time the page with the field loads:
PHP Notice: Trying to get property of non-object in /Users/.../wp-content/plugins/advanced-custom-fields-pro/api/api-helpers.php on line 1709
Seems like a bug.
