I was also having the same problem so I opened the header.php on the default theme and noticed that the update has changed
<h3 class="menu-toggle"><?php _e( 'Menu', 'twentytwelve' ); ?></h3>
to
<button class="menu-toggle"><?php _e( 'Menu', 'twentytwelve' ); ?></button>
so simply replace the code on the header.php of your child theme to reflect the updated one.
It has solved my problem but I would still love to hear from the developers.