Thanks both for your help.
I guess since I need to use this in the admin, not as a front-end form, then I cannot show a specific field based on the parent_id/parent? Would be nice to expand this functionality out in the future, as I guess it could become quite useful.
Here is my setup... I have a 'News page', with two child pages 'Jobs' and 'Funding'. These child pages act as categories/filters so users can view all news, just jobs, or just funding. Each page within these categories uses the same template, and has a field 'closing_date' and 'attachments' so if the page you are adding has 'Job' as a parent, then it should show these fields... otherwise not. So maybe I just create two templates... but just thought I would try to make it as global as possible.
Thanks again
R