Password protection

Hi,
I’m trying to protect with password a group of child pages under the same parent page. I am able to protect each and every page within the section, that means having a password entry for each page, but I would like to protect the parent page only with a password and be able to protect every child page from there (avoiding to insert a password every time you visit a child page). How can I do it?
..hope that make sense to you :-)
Thank you very much