How to get child style.css to load last rather than second behind parent style.css?

If the dependency array contains the IDs of all the other stylesheets, then yours should load last.

Try adding the other stylesheets to the array when you enqueue your stylesheet:

wp_enqueue_style ( 'child-style',
  get_stylesheet_directory_uri() . '/style.css',
  array ( $parent_style, 'bootstrap', 'bootstrap-touchspin' )
);