Page Children from Page with Custom Template arent displaying correct

Hello,
in my theme pages arent displaying correct when they are children of a page with a custom template.
My file structure is the following:

archives.php
footer.php
front-page.php
functions.php
header.php
index.php
my-custom-template.php
another-custom-template.php

Page structure is the following

Parent Page //my-custom-template.php applies here
 - Child Page 1
 - Child Page 2

When I now add the parent page in my main menu and the child pages as menu childrens of the parent page item and then click on the childrens menu item the front-page.php is loaded.
But the URL is correct (https://blogurl.tld/parent-page/child-page-1).

So how can I fix this?