Query to include children's iteration

We are keeping our customer commit stories in a "backlog" iteration while they are "active" and are creating children tasks to both our dev team to do the work and to the L3 support team to track any deliverables to a fixpack stream.

We want to see customer commit stories that do not have a child work item in a fixpack iteration to be able to tell which commitments don't look like they are being delivered so we can take appropriate action.

We created a query that finds all our customer commit stories AND Children->Planned For -> is not part of -> Iteration Fixpack 1 hopefully to show all stories that don't have a child task that is part of Fixpack 1. It is returning some stories though that do have children in Fixpack 1.s in Fixpack 1!

I think the query is doing is looking for is if a single child task is not part of instead of checking to see if all tasks associated with our story are not part of for our query to work. Is there a way to configure the "is not part of" to have this behavior? Maybe there is a better query we can write?