Adding my own custom woocommerce shortcode to child theme, how?

The global $woocommerce can be reached, you just have to write global $woocommerce; before using it. Read this article.

You can define your own shortcode, see the documentation. You can copy/paste the code because it’s wrapped in a class so it won’t trigger fatal error due to double declaration.

But shortcode do not work that way :

function shortcode_handler($atts) {
  //code goes here
 }
add_shortcode('name_of_shortcode','shortcode_handler');