SQL 2005 Nested Stored Procedure, how to return a value to a trigger

I am trying to write a stored procedure that I can call from a trigger that will return a comma separated ID list of the parent and it's children.  This is what I have for the stored procedure:

CREATE PROCEDURE dbo.ShowHierarchy
(
	@Root int,
    @idList varchar(100) OUTPUT
)
AS
BEGIN
	SET NOCOUNT ON
	DECLARE @IndID int, @IndName varchar(30)

	SET @IndName = (SELECT ID FROM dbo.forum_industries WHERE ID = @Root)
	PRINT REPLICATE('-', @@NESTLEVEL * 4) + @IndName
    
    SET @idList = @idList + ',' + CAST(@IndName AS VARCHAR(100))

	SET @IndID = (SELECT MIN(ID) FROM dbo.forum_industries WHERE ParentID = @Root)

	WHILE @IndID IS NOT NULL
	BEGIN

        EXEC dbo.ShowHierarchy @IndID, @idList--, @finalIdList
		SET @IndID = (SELECT MIN(ID) FROM dbo.forum_industries WHERE ParentID = @Root AND ID > @IndID)
           
    END
	
END

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When I call this, I do get in the message the right ID's displayed.  By the way, the @IndName is actually the ID and I will name it better once I get this working.

In my Trigger I am not sure how to call it and get the value back.  My trigger code

BEGIN
    DECLARE @IndID int
    DECLARE @idList varchar

	-- Find out whether we check the inserted or deleted table, depending on whether a record is getting deleted or inserted or updated.
	if (select count(*) from inserted) <> 0 and (select count(*) from deleted) = 0 --insert
	begin
		SELECT @IndID = forum_industriesID FROM inserted
	end
	if (select count(*) from inserted) <> 0 and (select count(*) from deleted) <> 0 --update
	begin
		SELECT @IndID = forum_industriesID FROM inserted
	end
	if (select count(*) from inserted) = 0 and (select count(*) from deleted) <> 0 --delete
	begin
		SELECT @IndID = forum_industriesID FROM deleted
	end
    
        
    SET @idList = 0
    EXECUTE ShowHierarchy @IndID, @idList
  -- Query code to store the IDList or select based on the list
END

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So, my hope is that my trigger will call this ShowHIerarchy and pass in the root, and return a value like 5,6,7,10,50,100  and I can use that in a query like: select count(id) from table where forum_industriesID IN (@idList)

The code I have so far returns nothing, so I don't know how to get the values back out.

I am fairly new to writing stored procedures and triggers and so any suggestions would be most helpful.