Best index for this scenario
Seems like the following would be sufficient:

CREATE TABLE dbo.tblCustomText
(
  ParentID int NOT NULL,
  CustomFieldID int NOT NULL,
  Value char(50) NOT NULL, 
  Log_CreateDate datetime NOT NULL,
  PRIMARY KEY CLUSTERED (ParentID, CustomFieldID)
);


You may also find later that you are querying by date, so you may want a non-clustered index on the Log_CreatedDate column.