There isn’t actually a GUI control for this (I should definitely add that!) but it’s easy to add with a snippet of PHP in the PHP tab of Chameleon:

if ($content_genius->check("ancestor_category_ids = 5,6,7")) return true;

What that does is that if any ancestor (higher level category) of the current category is 5 or 6 or 7, then make the rule succeed. i.e. all categories that descend from these ones will trigger the rule.

If you want to include the ancestor category itself as well, you can do this:

if ($content_genius->check("ancestor_category_ids = 5,6,7")
  or $content_genius->check("category_id = 5,6,7")
) return true;