You can try this. It might work for you but without more test data it's difficult to be sure.

DECLARE @TableA TABLE (ItemID INT, Date DATETIME)

DECLARE @TableB TABLE (ItemID INT, ParentID INT)

INSERT INTO @TableA

SELECT 1, '2012-09-08' UNION ALL SELECT 2, '2012-08-07'

UNION ALL SELECT 3, '2012-09-09' UNION ALL SELECT 4, '2012-09-10'

UNION ALL SELECT 5, '2012-08-23'

INSERT INTO @TableB

SELECT 1, 2 UNION ALL SELECT 3, 2

;WITH CTE AS (

SELECT b.ItemID, b.ParentID, a.Date

FROM @TableB b

INNER JOIN @TableA a ON b.ParentID = a.ItemID

UNION ALL

SELECT a.ItemID, NULL, a.Date

FROM @TableA a

)

SELECT a.ItemID, b.Date

FROM (

SELECT ItemID, Date, ParentID

,rn=ROW_NUMBER() OVER (PARTITION BY ItemID ORDER BY ParentID DESC)

FROM CTE

) a

INNER JOIN @TableA b ON a.ItemID = b.ItemID

WHERE rn=1

ORDER BY a.Date, ParentID, ItemID

Note that it only works for one level of parent-child hiearchy.