Help with T-SQL query

• I have 3 hierarchical tables. [Parent] ->[Child1]->[Child2]
• I have [Xref] table which stores the documents associated with these three tables. In the [Xref] table, the column “TypeID” defines to which table the document is associated with.
• Below is the table structure with some sample data.

Question: Given parented I want to get all the documents associated with parent as well its children. My sql query uses Union and its working fine. But Is there any other simple way to re-write this query?

(Note that this is just an example, the actual hierarchy is pretty long so my SQL is getting little messy and I want simplify it.)

TYPES
TypeID			TypeName
-----------------------------------------
1				Parent
2				Child1
3				Child2

TABLE - PARENT
ParentID	
-------------------------------------
1
2
3

TABLE - CHILD1
Child1ID	ParentID
----------------------------------------
1			1
2			1
3			2


TABLE - CHILD2
Child2ID	Child1ID
--------------------------------------------
1				1
2				1
3				2
4				2


XREF
DocumentID		TypeID		LocatorID
------------------------------------------
1				1				1
2				1				2
3				1				3
4				2				1
5				2				2
6				2				3
7				3				1
8				3				2
9				3				3
10				3				4

	SELECT DocumentID FROM XREF X
	JOIN Parent P ON P.ParentID = x.LocatorID AND TypeID = 1 -- Parent
	WHERE P.ParentID = @ParentID
UNION
	SELECT DocumentID FROM XREF X
	JOIN Child1 C1 ON C1.Child1ID = x.LocatorID AND TypeID = 2 -- Child1
	JOIN Parent P ON P.ParnetID = C1.ParentID
	WHERE P.ParentID = @ParentID
UNION
	SELECT DocumentID FROM XREF X
	JOIN Child2 C2 ON C2.Child1ID = x.LocatorID AND TypeID = 3 -- Child2
	JOIN Child1 C1 ON C1.Child1ID = c2.Child1ID
	JOIN Parent P ON P.ParnetID = C1.ParentID
	WHERE P.ParentID = @ParentID