I was thinking it would have to be done in the functions.php file in the child theme.
Turns out you can copy theme files such as header.php, footer.php, sidebar.php and others into your child theme folder, add the ARIA role where you want it, and that version of the file is what gets served. Updates to the parent theme don’t cause the ARIA roles to disappear.
Look at the beautiful ARIA roles on my blog now. And I have role=”contentinfo” in the footer, too!
Thanks to Automattic for running a Happiness Bar at Word Camps!