The master page can't directly call functions in the child class - it may not exist!
It's similar to the situation when you derive a C# class from a base class. The base class does not know what classes will be derived from it, so it can't call any of the methods in it, unless they are overridden versions of the methods already within the base class: it's like a generic Fruit trying to call a "Orange Segment" method on a Grape!
The chances are that wanting to do this is a symptom of a bad design choice.
There are a couple of was you can do it though if you must. Have a look here: http://stackoverflow.com/questions/887178/content-page-class-method-calling-from-master-page-class