Keep clean SimpleIoc initialization when reusing UserControl

Question

Originally I had ChildUserControl inside ParentUserControl inside MainWindow with this view-models (here only constructors):

public MainViewModel(IParentViewModel pvm) {}

public ParentViewModel(IChildViewModel cvm) {}

public ChildViewModel(IMessenger m) {}

And straightforward DI initialization in ViewModelLocator:

var container = SimpleIoc.Default;
container.Register<IMessenger, Messenger>();
container.Register<IParentViewModel, ParentViewModel>();
container.Register<IChildViewModel, ChildViewModel>();
container.Register<IMainViewModel, MainViewModel>();
...
public IMainViewModel MainViewModel => ServiceLocator.Current.GetInstance<IMainViewModel>();

Then I needed to add second ParentUserControl instance near the first one, and let them work independently. MainViewModel constructor became to look like this:

public MainViewModel(IParentViewModel pvm1, IParentViewModel pvm2) {}

But DI initialization changed dramatically:

container.Register<IMessenger, Messenger>();

container.Register<IParentViewModel>(
    () => new ParentViewModel(new ChildViewModel(container.GetInstance<IMessenger>("key1"))),
    "key1");

container.Register<IParentViewModel>(
    () => new ParentViewModel(new ChildViewModel(container.GetInstance<IMessenger>("key2"))),
    "key2");

container.Register<IMainViewModel>(() => new MainViewModel(
    container.GetInstance<IParentViewModel>("key1"),
    container.GetInstance<IParentViewModel>("key2")));

As you can see, now I have to point to concrete ParentViewModel and ChildViewModel classes in several places, and I also have to manually use GetInstance everywhere. I think this is not good.

What can you tell about this latter DI initialization? I guess it can't be improved by SimpleIoc, but can Unity or Ninject help in this situation?