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);