How do I highlight the menu for a child page?

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The archives.php template file is a custom page template file for a static Page. The is_archive() and is_home() conditionals apply to a blog posts archive and blog posts index, respectively.So, you’re never going to get is_archive() to return true while displaying the output of archives.php.

To have your archives.php static Page appear in the menu:

  1. Create a new static Page, named whatever, e.g. “Archives” (but it is completely arbitrary)
  2. Assign the archives.php custom Page template to this static Page
  3. Add this static Page to your custom Nav Menu

Then, you should see it highlighting properly in the menu.