Adding Code snippets to functions.php when using a child theme?

I have a chunk of code i need to add to the function.php. i'am using a 2011 child theme, which does not have a functions.php file. Should i just create a functions.php file for the child theme and add only the code snippet to it, or the entire 2011 function.php + the code snippet.

I have tried it both ways and get the blah blah blah in public_home/blah blah/ not; here let me get you the error message!

Fatal error: Cannot redeclare twentyeleven_setup() (previously declared in /home/tithi/public_html/domain.com/wp-content/themes/twentyteleven-w4f/functions.php:53) in /home/tithi/public_html/domain.com/wp-content/themes/twentytwelve/functions.php on line 77

Thanks!