Is there a way to add a NOT element/clause to a query

In RTC 4.0.1 I want to write several related queries where children exist but NONE of them match a certain condition. For example to find all items with children that have all completed chose "Children -> Status is not XXX" but this will be true if ANY of the children have completed (but others still may be in progress!!). I could imagine this being solved by a NOT(Children -> Status is XXX). Is this possible?
Thank you