/css/ie.css in childtheme wouldn't be loaded is because the fn call is not refering to childtheme's path, look around inside the
function.php. There is a function
twentytwelve_scripts_styles() which is added to
wp_enqueue_scripts action hook, this is to load the js(scripts) and css(styles) into the
* Loads the Internet Explorer specific stylesheet.
wp_enqueue_style( 'twentytwelve-ie', get_template_directory_uri() . '/css/ie.css', array( 'twentytwelve-style' ), '20121010' );
$wp_styles->add_data( 'twentytwelve-ie', 'conditional', 'lt IE 9' );
The above is a part of it that's responsible for the
ie.css, notice the
get_template_directory_uri() which is the path to parent theme, not the child's. Now if anyone want to make changes to this ie.css in childtheme he/she will have to make a function call like this in child's
function.php and use
get_template_directory_uri() = path to parent's theme
get_stylesheet_directory_uri() = path to theme in use
This is why 2012 is a complicated theme, it's advance - not easy to customize for web designers, but yes, the ninja level coders will always say it's easy.
2012 is a great theme by itself, but 2012 as a default theme is a different matter.