Ingo,
Welcome to the forum! The reason why the child class method is called is because when you do
((Parent) childReference).overriddenMethod()
polymorphism comes into effect. Everytime you access an instance method that is overridden, the version of the method which is called is the version defined in the type of the object, not the reference (and in this case the object is a child class object, so you end up calling the overriding version of the method.)
As to whether you can call the superclass version without modifying the child class code: Not when you are calling the method through a reference which is linked to a child class object. In order to call the overridden version you would need to use the super keyword in the child class code to achieve that.
I hope that helped.