Hierarchical URL in menu editor

Hi!
I’m using menu editor feature to create my website menu. I’ve a trouble: when I create a page hierarchy, it’s working in frontend, but URL’s are not hierarchical (they’re based on the page tree, which is not practical at all).
Any idea to have url like “http://www.mysite.com/parent_page/page/child_page”?
Any plugin or hacking code?

Thank you!