How to turn link of pages in page menu on or off e.g. turn parent off & child on

Hi All,
I'm not sure of the correct terminology. But I have a page menu with main parents, and then have made child pages underneath by putting them in the parent category.
I would like to link to only the children, and not the parents.
Obviously I need the parent pages to show in the page menu, but I don't want to be able to link into them as there will be no text there, I just want to like to the children, or 'grand'-children pages.
I may even on occassion not want to link to the children, but link to the grandchildren

Basically my question is can one switch the ability to link to a page in the page menu on or off?

Thank you very much,
Dean