Check for existence of a child IC in Siebel Data Map Source Search Spec

We have an external data structure that looks like this:

Parent 1

     - Parent Attrib1

     - Child 1

          - Child Attrib1

     Child 2

          - Child Attrib 2

...

In mapping the external parent to internal parent, we want a "Source Search Specification" that says:

IF [Parent Attrib 1] IS NULL AND <NO CHILD RECORDS EXIST>

Any idea how to implement "<NO CHILD RECORDS EXIST>" logic in the Siebel Data Map / Integration Map?

I've thought about the concept of "COUNT" and "EXISTS" but these don't seem to exist in the world of the Data Mapper - at least, they are not documented.

Thanks in advance!