In reply to tech_savvy:
You need to be careful when using the terms 'parent' and 'child'. In UVM, a 'parent' will instantiate a 'child', which is normally a completely different type. In this regard, what Nitish recommended is correct.
In your case, you are dealing with base classes and extended classes. In this case, there is no way to access any additional variables in an extended class when using a base class handle.
You can $cast the variable to a handle for the extended class, but I would recommend creating a new covergroup for the variables in the extended class.