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'
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'