Which is quicker: Select of existing row vs Update where no row exists?
Can you not use the MERGE statement added with SQL Server 2008 to "UPSERT" in one atomic operation?

DECLARE @FilterValue INT

;MERGE 
INTO ChildFilterTable AS CFT
USING  (your filter, source thing here)
                  ON (CFT.ChildID = ...)
WHEN MATCHED
    THEN update stuff
WHEN NOT MATCHED BY TARGET
    THEN insert stuff;