Using the child theme…Steps provided by user “lisabee”.
1. Re-updated the Permalink settings (otherwise the portfolio pages returned an error 404). I just went into the Permalink under the Settings menu and clicked the Update Setting button and the portfolio pages returned.
2. Re-set the Main menu in the Menus option. The navigation menu that I had previously configured was still there but the Main menu setting was reset to blank when I switched to the child theme.
3. Copied the boxed the image directory [from the parent] to the child theme directory.