There's a fairly clear instruction to this in the functions.php of the 'twentytwelve' theme. and that is
* When using a child theme (see http://codex.wordpress.org/Theme_Development and
* http://codex.wordpress.org/Child_Themes), you can override certain functions
* (those wrapped in a function_exists() call) by defining them first in your child theme's
* functions.php file. The child theme's functions.php file is included before the parent
* theme's file, so the child theme functions would be used.
In the case of a menu item; all you need to do is save a new file as functions.php in your child and add
'name' => 'Banner Ads',
'id' => 'bannerads',
that's enough. as you're adding to a function .
Check Add functions to your wordpress child theme for more information.