How to change twentytwelve child theme site title separator

From this comment:

I know the title could be defined in the header.php file as but I din’t want to touch the existing header.php, but do it in a child theme’s functions.php instead

But Twenty Twelve isn’t modifying the separator in its wp_title filter callback; rather, it is defining it in its call to wp_title(), in header.php:

<title><?php wp_title( '|', true, 'right' ); ?></title>

Note that the wp_title callback merely passes $sep along, without modifying it:

function twentytwelve_wp_title( $title, $sep ) {
    global $paged, $page;

    if ( is_feed() )
        return $title;

    // Add the site name.
    $title .= get_bloginfo( 'name' );

    // Add the site description for the home/front page.
    $site_description = get_bloginfo( 'description', 'display' );
    if ( $site_description && ( is_home() || is_front_page() ) )
        $title = "$title $sep $site_description";

    // Add a page number if necessary.
    if ( $paged >= 2 || $page >= 2 )
        $title = "$title $sep " . sprintf( __( 'Page %s', 'twentytwelve' ), max( $paged, $page ) );

    return $title;
}
add_filter( 'wp_title', 'twentytwelve_wp_title', 10, 2 );

You have two choices:

1. Replace header.php via Child Theme, and call:

   <title><?php wp_title( '-', true, 'right' ); ?></title>
   

2. Remove Twenty Twelve’s filter callback, and replace it with your own:

   function wpse87673_filter_wp_title( $title, $sep ) {
       // Your callback code goes here
   }
   add_filter( 'wp_title', 'wpse87673_filter_wp_title', 11, 2 );
   remove_filter( 'wp_title', 'twentytwelve_wp_title', 10, 2 );
   

I don’t think there is a more simple way to filter $sep directly. It is defined by the wp_title() function call parameter.