This can be a complicated issue and it's called relational division. You can search the web about it and you'll find lots of information to learn about it.

Your sample data had an error on product 102, but this should do the trick.

DECLARE @ParentTable TABLE (ProductID BIGINT,ItemID BIGINT);

DECLARE @ChildParentTable TABLE (ParentID BIGINT,ProductID BIGINT,ItemID BIGINT);

INSERT INTO @ParentTable(ProductID,ItemID) VALUES(101,1234),(101,1578),(101,1590),(101,1237);

INSERT INTO @ParentTable(ProductID,ItemID) VALUES(102,5465),(102,5466),(102,5474),(102,5489),(102,6543);

INSERT INTO @ChildParentTable(ParentID,ProductID,ItemID) VALUES(101,701,1234),(101,701,1590),(101,701,1578);

INSERT INTO @ChildParentTable(ParentID,ProductID,ItemID) VALUES(102,901,5465),(102,901,5474),(102,901,8976);

SELECT ISNULL( x.ParentProductID, c.ParentID) AS ParentProductID,

ISNULL( x.ProductID, c.ProductID) AS ProductID,

ISNULL( x.ItemID, c.ItemID) AS ProductID,

CASE WHEN x.ParentProductID IS NULL THEN 1 ELSE 0 END AS IsExtra,

CASE WHEN c.ParentID IS NULL THEN 1 ELSE 0 END AS IsMissing

FROM @ChildParentTable c

FULL

JOIN (

SELECT DISTINCT

p.ProductID AS ParentProductID,

c.ProductID AS ProductID,

p.ItemID

FROM @ChildParentTable c

JOIN @ParentTable p ON p.ProductID = c.ParentID) x ON x.ParentProductID = c.ParentID

AND x.ProductID = c.ProductID

AND x.ItemID = c.ItemID

WHERE x.ParentProductID IS NULL

OR c.ParentID IS NULL

ORDER BY ParentProductID;