Here's a rCTE to get you started:
;WITH rCTE AS (
SELECT
[Level] = 1,
tr.ParentItemID, tr.ChildItemID,
List = CAST(CAST(tr.ParentItemID AS VARCHAR(10)) + '>' + CAST(tr.ChildItemID AS VARCHAR(10)) AS VARCHAR(8000))
FROM #tblItemRelationship tr
WHERE ParentItemID = 1
UNION ALL
SELECT
[Level] = lr.[Level] + 1,
tr.ParentItemID, tr.ChildItemID,
List = CAST(lr.List + '>' + CAST(tr.ChildItemID AS VARCHAR(10)) AS VARCHAR(8000))
FROM rCTE lr
INNER JOIN #tblItemRelationship tr ON tr.ParentItemID = lr.ChildItemID
)
SELECT *
FROM rCTE
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden