[Hampshire] xubuntu su <>sudo question

I am setting up an old laptop for my son which is running xubuntu 9.04. I don't
want him to be able to "sudo" so I set a password for su, I typed

sudo passwd

I then typed in a password which was accepted, now if I type su in a terminal
screen it asks for the above password and root access is obtained. But if as a
normal users if go into add\remove (programs) it will ask for a password for
the users and by entering the users password it gives me root access, if I try
the su (root) password I setup it gets rejected. How can I change this??

Tim