Determine the last child of the parent menu WP Nav Walker

The code below is not tested but is a good start for what you need.
You will need to have the Walker class declared on you theme and then extended on your below function .

class My_Walker_Nav_Menu extends Walker_Nav_Menu {
  function start_el(&$output, $item, $depth, $args) {
   ...

  if($args['has_children']) {
  // I know what to do here


  }
 }
}