Thank you.
So bender_child or benderchild should be the folder not bender-child.
That put the child in admin.
Now the problem is that on preview the page is blank:)
EDIT.
It seems functions.php must not be copied entirely but must contain only the new added functions.
After reading the threads until the end I decided to not use child theme
To many modifications needed...
Thank you, I didn't found that topic before I posted this topic.