functions.php

I want to have a child functions.php and when I copy the functions.php to the child folder, the site breaks. I tried just putting the modifications in the child folder functions.php assuming that it would be added to the standard functions.php in the main theme folder when the page is loaded. No luck.

How can I modify the functions.php and not lose my edits when the main theme is updated? Other than this problem, I like the Child Wizard.