Thank you all so much for your really fast support! What an amazing community this is!
Yesterday evening I came up with
if($page->status === 8193 || $page->status === 10241) ...
as a temporary solution, because I wasn't aware of
$page->isTrash
Thanks, Soma!
At the end, I followed ryan's advice:
I'll stick with $page->title, because $page->title is anyway needed when the new pages are created. In my case, that's not problematic, because the four child pages should always have the same names/titles, without any spaces or special characters.
The reason why I set up this module is: I have a client who needs a bunch of microsites. Every microsite has the same pages (home, imprint, contact... etc.). Because some of the placeholders are used on one or more of these pages, it's more comfortable for the client just to edit one page (the parent of the four childs). With this plugin, the client has not to take care of creating the child pages himself (in fact, I have set appropriate permissions, therefore he even can't create or edit child pages). Now, he is able to create all of the microsites with just one form - with the help of ProcessWire! Great solution in just two days altogether. I like this system so much, I'm sure it will have a bright future.