One possible solution is to put the Parent field at the parent part, rather the sub-query:



     SELECT Name, Id, ( SELECT Id, Title, RecordType, LastModifiedDate,
     CreatedBy.Name, ParentId from CombinedAttachments WHERE ParentId =
     '001b0000009ovxS' ORDER BY LastModifiedDate DESC limit 100 ) FROM
     Account WHERE Id = '001b0000009ovxS'