Isn't the code in that link another way of setting a child package variable to the get its value from the parent package? The same thing you can do with using a configuration and setting the parent package variable to load the child.

I've been testing how to run the same package with different variables, my screenshot should help explain. In each script (Not the controller) I change the cfg_ExtractType_Parent value to a different value, either "PB_DAILY", "HB_DAILY", or "HB_ACCOUNTS".

Then in the child task I bind the variable coming in to an email subject and mail it to me. I end up with three emails with the subject "HB_ACCOUNTS"