You are so welcome. Glad you've manged it to work. One mistake I made in my last response, what I mean is a cascading dropdown field (this applies for both parent field and child field) has changing 'ID' but static 'name'.
Sorry but there's no documentation on this. What I suggest is to install an add-on called firebug into your Firefox browser and use it to track form elements and debug your code. After opening firebug, you can simply focus on any object and check the name, ID or any other attributes of it (see screenshot), then decide on how to refer to the element in your code.