Plugin to restrict who can edit specific pages

Help! I posted this question to the “How Tos” forum, but it probably belongs here.

We’re using WordPress as a CMS solution, and our site includes many parent pages and child pages. How can I restrict a user to editing only certain parent pages (and their child pages)?
E.g. allow a user to edit all child pages under mysite.com/parentA/ but not mysite.com/parentB/ or mysite.com/parentC/.

Most of the plugins I’ve read about are either out of date (Role Scoper, User Access Manager) or require pricier pro versions to restrict editing to specific pages (Press Permit Pro, User Role Editor). The pro versions offer more than I need, I just want to keep the out-of-the box WP roles, except specify which pages an “editor” can edit.

Is there a feature within WordPress itself that I’m missing, or is this something that has to be done through a plugin?

The site for which I want to limit editor access to certain pages is part of a WordPress Multisites install.

Thanks so much for any help or advice you are able to provide!