how to get only grandchild category from a child category

One way you could do it is to make another get_categories inside the foreach ( $categories as $category )

Example code

   //preparing an array to hold later info
    $grandchildren_ids = [];
    //getting the child categories of parent 72
    $args = array(
     'orderby' => 'name',
     'parent' => 72,
     'taxonomy' => 'category',
     'hide_empty' => 0
    );
    $categories =  get_categories($args);

    foreach ( $categories as $category ) {
    //setting up the args where the parents are the child categories
     $grandchildrenargs=array(
     'parent' => $category->term_id,
     'taxonomy' => 'category',
     'hide_empty' => 0
     );
     $grandchildrencategories =  get_categories($grandchildrenargs);

      foreach ( $grandchildrencategories as $grandchildrencategory ) {
       //getting the grandchildren ids or whatever else is needed and populating the array
       $grandchildren_ids[] = $grandchildrencategory->term_id;
      }

     }
    var_dump($grandchildren_ids);