get_pages – grandchildren only

hello

i am trying to use get_pages to display only the grandchildren of 1 page. any help is greatly appreciated. thank you

<?php
	$args = array(
		'post_type' => 'page',
		'post_status' => 'publish',
		'child_of' => 8
		);
	$attachments = get_posts($args);
	if ($attachments) {
		foreach ($attachments as $post) { ?>

			<div class="portfolio_grid_thumbnail"><a href="<?php the_permalink(); ?>"><?php the_post_thumbnail('100,100') ?></a></div>

			<div class="portfolio_grid_title"><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></div>

		<?php
		}
	}
?>