Added 2016/02/05: There's a newer version of this post with very detailed steps to show you how to set the URL structure for this use case right.
This gist is kinda of my inspiration to create Completely Delete. Last year a client project need to cross reference 2 different post types. When I do some research on Google, a post from My Musings describe how to change the parent select options to posts from another post type. This is quite useful and a time saver for me.
After the cross reference work had been done, the client found another issue that when he trash or delete the parent post, the child posts won’t be trashed / deleted with it.
Few months later when I planed to develop my first plugin for the new business – 1Fix.io, I decided to fix this issue as a start point.