Parents, Children & Grandchildren – How do you call grandchildren of a…

I have this code, which makes all pages that are children of pageid 1639 display a certain template. I want to change it so it will work for grandchildren for for pageid 1639 as well.

This is what I currently have:

<?php if (is_page(1639) || $post->post_parent=="1639"): ?>
<?php get_template_part( 'loop', 'about' ); ?>

Thanks in advance