Hi @ussailing
This is a very complex request and not one that I can fully code for you.
You can use a post_object field to select a post. Then, in your template, you can display the child page links with the following code:
<?php
$selected_post = get_field('selected_post');
wp_list_pages( array(
'child_of' => $selected_post->ID
));
?>
ref: http://codex.wordpress.org/Function_Reference/wp_list_pages
You will need to write your own logic to determine if no children exist.
hope that helps.
Thanks
E