Woocommerce – Get product category 2nd level category

Assuming you know the parent category ID, you can use get_terms() function.

$args = array(
     'parent' => 100 // id of the direct parent
);

$cats = get_terms( 'product_cat', $args );

foreach( $cats as $cat ) {
  echo $cat->name;
}

Here is more information on this function and additional arguments you can use.
http://codex.wordpress.org/Function_Reference/get_terms