>>SELECT * FROM ChildTable ct WHERE NOT EXISTS (SELECT 1 FROM ChildTable WHERE pk = ct.pk and SomeDate > ct.SomeDate) >>>
SELECT * FROM ChildTable ct WHERE NOT EXISTS (SELECT 1 FROM ChildTable WHERE FK = ct.FK and SomeDate > ct.SomeDate)use the field you need to aggregate on here to join, e.g. ClientID in your case.
select * from ChildTable where PK = (select top 1 PK from ChildTable order by ModDate DESC)