Mandatory sub-elements of the observable collection

I have the following viewmodel which I'd like to bind to a view:

public class EntityManagerModel
    public readonly ObservableCollection<EntityViewModel> m_Entities =
             new ObservableCollection<EntityViewModel>();
// implements INotifyPropertyChanged
public class EntityManagerViewModel: BaseViewModel
  private EntityManagerModel m_Model = new EntityManagerModel();

  public ObservableCollection<EntityViewModel> Entities
      get { return m_Model.m_Entities; }

However what I actually would like to bind to my view is the data contained in EntityViewModelwhich is defined like so:

public class EntityModel
   public readonly PointCollection m_TrailPoints = new PointCollection();
// implements INotifyPropertyChanged
public class EntityViewModel: BaseViewModel
   private EntityModel m_Model = new EntityModel;

   public PointCollection TrailPoints
     get { return m_Model.m_TrailPoints; }

So, I'd like my view to update every time I add a point like this:

// In some routine inside EntityManageViewModel I have this code
// this should trigger collectionChanged event:
Entities[index].TrailPoints.Add( new Point(x, y));

My view's XAML code is:

<!-- EntityManagerView -->
    <!--Trail line-->
    <ItemsControl ItemsSource="{Binding Entities}">
                <Canvas />

            <DataTemplate DataType="{x:Type vm:EntityViewModel}">
                <Polyline Stroke="Red" StrokeThickness="1">
                        <Style TargetType="{x:Type Polyline}">
                            <Setter Property="Points">
                                    <MultiBinding Converter="{StaticResource pointMultiConverter}">
                                        <Binding Path="ActualWidth" RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type Canvas}}"/>
                                        <Binding Path="ActualHeight" RelativeSource="{RelativeSource FindAncestor, AncestorType={x:Type Canvas}}"/>
                                        <Binding Path="TrailPoints"/>

This only displays/updates the lines when i resize the application window. Of course, I have found solutions like this while searching. But subclassing ObservableCollection introduces other problems in my code. So, my question is what is the best/alternative way to do this?

The easiest solution to "simulate" an observable collection is to add the following line to the constructor of your EntityViewModel:

public EntityViewModel()
    TrailPoints.Changed += (sender, args) => RaisePropertyChanged("TrailPoints");

Whenever the TrailPoints collection is changed (e.g. when a Point is added), its Changed event is raised. The idea is to subscribe to this event and raise the PropertyChanged event to inform the View about the collection has changed!