How To Convert Recursive Function Into Recursive Stored Procedure

I am having problem to apply updates into this function below. I tried
using cursor for updates, etc. but no success. Sql server keeps telling
me that I cannot execute insert or update from inside a function and it
gives me an option that I could write an extended stored procedure, but
I don't have a clue of how to do it. To quickly fix the problem the
only solution left in my case is to convert this recursive function
into one recursive stored procedure. However, I am facing one problem.
How to convert the select command in this piece of code below into an
"execute" by passing parameters and calling the sp recursively again.

### piece of code ############
SELECT @subtotal = dbo.Mkt_GetChildren(uid, @subtotal,
@DateStart, @DateEnd)FROM categories WHERE ParentID = @uid

######### my function ###########
CREATE FUNCTION Mkt_GetChildren
(@uid int, @subtotal decimal ,@DateStart datetime, @DateEnd datetime)
RETURNS decimal
AS
BEGIN

IF EXISTS (SELECT
uid
FROM
categories WHERE
ParentID = @uid)
BEGIN
DECLARE my_cursor CURSOR FOR
SELECT uid, classid5 FROM categories WHERE parentid = @uid

declare @getclassid5 varchar(50), @getuid bigint, @calculate decimal
OPEN my_cursor
FETCH NEXT FROM my_cursor INTO @getuid, @getclassid5
WHILE @@FETCH_STATUS = 0
BEGIN
FETCH NEXT FROM my_cursor INTO @getuid, @getclassid5
select @calculate = dbo.Mkt_CalculateTotal(@getclassid5, @DateStart,
@DateEnd)
SET @subtotal = CONVERT (decimal (19,4),(@subtotal + @calculate))

END
CLOSE my_cursor
DEALLOCATE my_cursor

SELECT @subtotal = dbo.Mkt_GetChildren(uid, @subtotal,
@DateStart, @DateEnd)FROM categories WHERE ParentID = @uid
END
RETURN @subtotal
END
GO

Rod