select last child passing by parent id sql server

ALTER function [dbo].[fn_LastChild_Select]
(
	@ParentId int
)
returns int
as
BEGIN

	Declare @childid int
	Declare @Id int
	SET @Id=@ParentId
	
	WHILE EXISTS(select * from dbo.miscDemographics_Relation where miscDemographics_MasterId=@ParentId)
	BEGIN
	
       SELECT @ParentId=miscDemographics_Id,@childid=miscDemographics_Id  FROM dbo.miscDemographics_Relation where 
       ISNULL(miscDemographics_MasterId,0)=@ParentId
       
	END
	
	SET @childid=ISNULL(@childid,@Id)
	
	return @childid
END