private void Button_Click(object sender, RoutedEventArgs e)
    {
        //Do not do this, example only
        var f = new First("Billy");

        mvm.AddFirstChild(new FirstViewModel(f));
        //Prove that the event was raised in First, FirstViewModel see & handles it, and
        //the UI is updated
        f.AddChild(new Second(int.MaxValue));
    }