reponsive iframes

I created a child theme of the Responsive Theme by cyberchimps so that I could add custom css (style.css)

…now I would like to edit the functions.php file.

Is it possible to copy the functions.php file to a child theme of Responsive to add a function to make iframes responsive?

I see two functions.php files, one within theme folder (wp-content/themes/responsive) and one within responsive folder (wp-content/themes/responsive/includes).

Which one to edit within the child theme?

Anyone else try making iframes responsive?

_Cindy