I uploaded a file to /my-child-theme/wpforo/wpf-includes/class-members.php for trying to customize class-member.php, but it doesn't change.
If change the class-member.php of the core plugin directly, it will be reflected.
Is the file upload location correct when customizing?
I referred to this document page.
You can customize only active theme folder files, Theme files can be found within the /wpforo/wpf-themes/ directory, in current active theme folder, for example /classic/.
In your case, you are trying to customize wrong folder files.
And please read the documentation attentively: