KaeruCT

This plugin allows passwords for password-protected pages/posts to be passed directly through the URL.
The query string parameter that should contain the password is <code>pw</code>.
For example, if the URL of your post is <code>http://myblog.com/password-protected-page/</code> and the password is <code>PASSWORD</code>, 
then just append <code>?pw=PASSWORD</code> to it.
If the URL already contains a query string (for example, <code>http://myblog.com/?p=5</code>), then be sure to append <code>&pw=PASSWORD</code> instead.

Password Passthrough

This plugin allows passwords for password-protected pages/posts to be passed directly through the URL.

fullthrottledevelopment

Glenn Ansley

This plugin does one thing. If a page that is password protected has children pages, all children pages will be protected with the same password.
If the correct password is entered on the parent page or any of its children pages, all related pages will be viewable to the user.
The plugin protects unlimited levels of grandchildren pages via the $post->ancestors. It uses first ancestor that is password protected.
<h3>Additional Information</h3>
The plugin currently works by looking for parent pages that are password protected and applying the same restrictions the the currently being viewed child page. This means that the children pages do not actually get a password added to the database.
I’m trying to decide if this is the best way to proceeed or if I should take another route (such as adding / updating / removing passwords from children pages at the write or save post screen).
Please feel free to offer any suggestions or report any bugs here: [http://fullthrottledevelopment.com/password-protect-children-pages/]
Thanks!

FT Password Protect Children Pages

Applies the same protection to children pages applied to the parent page.

Password Passthrough vs FT Password Protect Children Pages