Hope this will help you who want to implemet a sub/child post such as child page in your wordpress.
WP Sub Post is a wordpress plugin that allow you to make your wordpress post has a parent or child post.
* Child post is a normal post that has a parent.
* Child post only displayed on it’s parent post page or when parent page is displayed.
* Child post can not be viewed individually.
* Child post has all possibility like a normal post, such as attachment, custom fields etc.
This plugin is still in development. so, your feedback and idea for improvisation are welcome...
More information and download can be found here...