put it in the folder “includes” of my child theme
Don’t put it there. Just put it in the child theme folder and see if that works.
BTW I recently update the Graphene theme, should I update my child theme’s files?
The child theme folder should only contain files you create, so there wouldn’t be anything to update unless you have edited one of your own files. You didn’t copy the entire theme over to this folder, did you?
I made a custom file “theme-slider.php”
I’m curious. What changes are you trying to make?