Change Footer Credits – Help with child theme Functions

Hi again everyone,

I’m getting confused about where to place the following code to alter my footer credits:

<?php

add_filter('tc_credits_display', 'my_custom_credits');
function my_custom_credits(){
$credits = 'Developed by Polly Philipson';
$newline_credits = '';
return '
<div class="span4 credits">
                    <p> &middot; &copy; '.esc_attr( date( 'Y' ) ).' <a href="'.esc_url( home_url() ).'" title="'.esc_attr(get_bloginfo()).'" rel="bookmark">'.esc_attr(get_bloginfo()).'</a> &middot; '.($credits ? $credits : 'Developed by <a href="https://www.pollyphilipson.com/">Polly Philipson</a>').' &middot;'.($newline_credits ? '<br />&middot; '.$newline_credits.' &middot;' : '').'</p>        </div>';
}

?>

Is the code correct and how do I create a functions.php file in my child theme?

Any help much appreciated!!