Get related post from some selected pages?

I have many subpage under an parent page , in these subpages, I want to get related posts from these subpages, how to get it ?
Maybe this will help: https://www.ads-software.com/extend/plugins/better-related/installation/ , use WP_Query with post_type=page&parent_post=page_id argument can query the given subpages.

Expecting for reply.

Finally, this plugin is very great.

https://www.ads-software.com/extend/plugins/wordpress-23-related-posts-plugin/