[Twenty Twelve] functions.php for modifying header size

I got this snippet from here on the forums.

<?php
function twentytwelvechild_custom_header_setup() {
	$header_args = array( 'height' => 550, 'width' => 950 );
	add_theme_support( 'custom-header', $header_args );
}
add_action( 'after_setup_theme', 'twentytwelvechild_custom_header_setup' );
?>

Site is here.

It doesn’t seem to be loading, or at least I see no trace of it when I use Google to inspect the code. I have a header.php in the child theme as well as this function.php and style.css. Is it being over-ridden?

Edited to ask:
I see the settings I want in a function in custom-header.php in the parent theme. Is THAT overriding the functions.php in the child?

Should I not just create a custom-header.php in the child theme folder?