Here's a simple solution that works :

Code-behind

public partial class Window1 : Window
{
    public Window1()
    {
        InitializeComponent();
        var dataSet = getData();
        _rootNodes = dataSet.Tables["data"].DefaultView;
        _rootNodes.RowFilter = "ParentId IS NULL";
        this.DataContext = this;
    }

    private DataView _rootNodes;
    public DataView RootNodes
    {
        get { return _rootNodes; }
    }

    internal static DataSet getData()
    {
        ...
    }

}

XAML

    <TreeView ItemsSource="{Binding RootNodes}">
        <TreeView.ItemTemplate>
            <HierarchicalDataTemplate ItemsSource="{Binding rsParentChild}">
                <TextBlock Text="{Binding NodeDescription}" />
            </HierarchicalDataTemplate>
        </TreeView.ItemTemplate>
    </TreeView>