Its been a long day so test the sql in your favorite query tool to verify it =P
But your SQL is something like this, you should be able to use this to generate a predicate..
SELECT *
FROM dbo.Parent P (nolock)
LEFT JOIN dbo.Child C (nolock) on C.ParentKey = P.ParentKey
WHERE C.ParentKey is null -- can actually be any field in the child since all fields return null
Just add the relation with a left hint and the compare predicate.
John