Editing the functions.php file in a child theme

Just create one. Add the <?php tags in the top, and ?> at the bottom and put your code between them.

Don’t edit functions.php of the parent theme. Make a new functions.php in the child directory and use it for your custom code. For more info, see Child Themes, specifically: “The functions.php of a child theme does not override its counterpart from the parent. Instead, it is loaded in addition to the parent’s functions.php. It is loaded right before the parent’s file.”