You can initialize the $scope to be whatever you want when you test your inner controller so you can mock out what the parent controller would have set on it
var controllerInstance;
beforeEach(inject(function ($controller, $rootScope) {
scope = $rootScope.$new();
scope.myReferencedObject = {someProperty: 'hello world'}
controllerInstance = $controller('childController', {
$scope: scope
});
}));