Permalink child pages or posts, not both

I have a WordPress site running on IIS 7 with the Rewrite functionality installed.

In the WordPress Permalink settings I change to /%category%/%postname% and everything works except child pages. Well, and Archive links, but I suspect that’s a separate issue.

If I do a global $wp_rewrite; $wp_rewrite->flush_rules(); in functions.php, as I’ve read can help, it will fix child pages, but break posts.

If I save the Permalink settings again, it switches back.

Here’s the rewrite rule I’m using.

<rewrite>
  <rules>
    <rule name="WordPress Rule" stopProcessing="true">
      <match url=".*" />
      <conditions logicalGrouping="MatchAll">
        <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
        <add input="{REQUEST_FILENAME}" matchType="IsDirectory" negate="true" />
      </conditions>
      <action type="Rewrite" url="index.php?page_id={R:0}" />
    </rule>
  </rules>
</rewrite>

Any advice? Thank you.