code is here
you make a page template, and use that page template for your parent page, which is just a redirect. i like this neat redirect style, not plugin based, and your url still has the right heirachy but you dont end up on any boring inbetween pages, or have to use jquery to open and close the menu.
this is a function for use thats a bit smarter, for wider use, such as doing a certain thing if your a child of a certain branch of content...