Wordpress - Add this filter to your functions.php file to allow private and draft pages to be selected as parent pages.

functions.php

function allow_private_parent_pages_nh( $dropdown_args, $post = 0 ) {
	$dropdown_args['post_status'] = array('publish', 'draft', 'private');
	return $dropdown_args;
}
add_filter('page_attributes_dropdown_pages_args', 'allow_private_parent_pages_nh', 10, 2);
add_filter('quick_edit_dropdown_pages_args', 'allow_private_parent_pages_nh', 10, 2);