code: func (fb *Firebase) ChildChanged(fn ChildEventFunc) error {
  	return fb.addEventFunc(fn, fn.childChanged)
  }
docs: // ChildChanged listens on the firebase instance and executes the callback
  // for every child that is changed.
  //
  // You cannot set the same function twice on a Firebase reference, if you do
  // the first function will be overridden and you will not be able to close the
  // connection.