Bug or poor design? Changing parent page visiblity

We are having the following problem with our wordpress install.

The process:
1. A parent page is created
2. Child pages added to this parent page.
3. The parent page is then set to either 'Private / draft or scheduled'
4. The child pages lose their parent

Why is this a problem?
When a parent is set to any of the above ALL child pages suddenly start appearing in the main menu of dynamic menus using wp_nav_menu()

Why are we doing this?
We want to add a LOT of time-released material so ideally have the parent page hidden from the main menu until X date.

The only work around we have seems to be setting ALL child pages to 'scheduled' as well which is incredibly laborious.

Basically, this MUST be a bug i.e. setting a parent page to anything other than 'Published' causes all child pages to lose their 'parent' ID. No logical person would deliberately design a system like that.
Is there a work around? Anyone else noticed that problem.

Thanks for reading and any help would be appreciated.

Alex Furr, University of Southampton