Page Not Found: Sub-Pages under one parent page

I got a “Page Not Found” error for all three sub-pages under a particular parent page.

My website is: https://www.aktranslation.co.nz.
The parent page is: certified translation
The three sub-pages that have problems are:
Driver Licence Translation
Police Certificate Translation
Immigration Documents.

All other pages (parent pages & sub-pages) are fine. And I can place the three pages under any other page, without any problem. The problem was not solved when I changed the theme to the default one. Therefore, it seems not to be a theme problem.

Any help would be great appreciated. If you need more information, please let me know. Thanks in advance!