As with most Google Apps features, setting page-level permissions within Sites is both simple to use and slightly unintuitive to access. To use it, first enable Sites on your Apps account. Then access your Sites URL (sites.google.com, by default). To enable page-level permissions, first go to a page that you would like to edit permissions for, and then click More actions in the upper-right corner, then click Sharing and Permissions.
Google Sites Adds Page-Level Permissions: First Look
Enable Sharing And Permissions
Enable Page-Level Permissions
Set Individual Page Permissions