You need to "export" the class. CoffeeScript uses an IIFE wrapper to avoid polluting the scope.
in your application.js :
#= require coffee_parent
#= require coffee_child
in your coffee_parent.js.coffee :
@Parent = class Parent
doStuff: ->
in your coffee_child.js.coffee :
@Child = class Child extends @Parent
doStuff: -> super 1