Post format selector in Thematic child theme post class

Okay, I’m going on the assumption that the real question is “how do I get Thematic to add the post-format to its body classes?”

Try this in your functions.php:

function my_thematic_post_format_class( $classes = array() ) {
  $format = get_post_format();
  if ( '' == $format )
    $format="standard";

  $classes[] = 'format-' . $format;

  return $classes;
}

add_filter( 'post_class', 'my_thematic_post_format_class' );

Don’t override the thematic post class functions. Just add that filter, and you should be good.