Transact SQL :: NVARCHAR SortOrder Breaks For More Than 9 Items?

I have below sample data table,

DECLARE @TBL TABLE (ItemId INT IDENTITY(1,1), ItemName NVARCHAR(20), ItemDate DATE, ParentItemName NVARCHAR(20), ItemOrder INT, ReportId INT)
INSERT INTO @TBL (ItemName, ItemDate, ParentItemName, ItemOrder, ReportId)
VALUES ('Plan', '2015-06-01', NULL, 1, 20),('Design', '2015-06-01', NULL, 2, 20),('Test', '2015-06-20', NULL, 3, 20),('Complete', '2015-06-30', NULL, 4, 20),
('Design child A', '2015-06-02', 'Design', 1, 20), ('Design child B', '2015-06-01', 'Design', 2, 20),

[Code] ....

Here I want,

1. to display all parent with ORDER BY ItemOrder (no need to sort by ItemDate)

2. display all child row right after their parent (ORDER BY ItemOrder if ItemDate are same, else ORDER BY ItemDate)

3. display all grand child row right after their parent (ORDER BY ItemOrder if ItemDate are same, else ORDER BY ItemDate)

Below Query works perfect, when count of child or grand child count less than 10, as SortOrder column here is

NVARCHAR,
;With cte As
(Select t.ItemId, t.ItemName, t.ItemDate, t.ParentItemName, t.ItemOrder, t.ReportId,
Cast(t.ItemOrder As nvarchar(max)) As SortOrder
From @TBL t
Where t.ParentItemName Is Null

[Code] ....

The output rows with ItemOrder's 10, 11, 12 (test grand child 10, test grand child 11, test grand child 12) should display after ItemOrder = 9 (test grand child 9). I know this is happening due to varchar sort order?