That is a very good question, and does reveal a blind spot in the current range of Applescript task properties.
At the moment, you can begin, in Applescript or Where in OF, with something like:

tasks where its due date < soon or (its parent task is not missing value and due date of its parent task < soon)

However, a task may, of course, be nested to an arbitrary depth within enclosing action groups, so this kind of query soon grows to be unwieldy, and there is, in any case, no way of directly detecting just how deeply nested a task is.

The cache does, however, track the timestamp value effectiveDateDue so it might be worth putting in a request for this to be exposed by a new Applescript property effective due date (analogous to effectively hidden which was very helpfully added this summer).

--