Роман

public class TreeNode
    {
        public int ParentId { get; set; }
        public string Name { get; set; }
        public ObservableCollection<TreeNode> Children { get; set; }
    }

    public class TreeViewModel
    {
        public ObservableCollection<TreeNode> Nodes { get; set; }

        public TreeNode SelectedNode { get; set; }
    }

    public partial class MainWindow : Window
    {
        public TreeViewModel model { get; set; }

        private int i = 1;

        public MainWindow()
        {
            InitializeComponent();

            model = new TreeViewModel();

            model.Nodes = new ObservableCollection<TreeNode>();
            this.DataContext = model;

            model.Nodes.Add(new TreeNode() { Name = "Root", Children = new ObservableCollection<TreeNode>() });
            model.Nodes.FirstOrDefault().Children.Add(new TreeNode() { Name = "Second" });

        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            if (model.SelectedNode != null)
            {
                model.SelectedNode.Children.Add(new TreeNode() { Name = "Children "+i.ToString(), Children = new ObservableCollection<TreeNode>() });
            }
            else
            {
                model.Nodes.Add(new TreeNode() { Name = "Root "+i.ToString(), Children = new ObservableCollection<TreeNode>() });
            }
            i++;
        }

        private void TreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
        {
            model.SelectedNode = (TreeNode)e.NewValue;
        }
    }

<Grid>
        <TreeView HorizontalAlignment="Left" Height="337" Margin="42,34,0,0" VerticalAlignment="Top" Width="303" ItemsSource="{Binding Path=Nodes}">
            <TreeView.ItemTemplate>
				<HierarchicalDataTemplate ItemsSource="{Binding Children}">
					<TextBlock Text="{Binding Name}" />
				</HierarchicalDataTemplate>
			</TreeView.ItemTemplate>
        </TreeView>
        <Button Content="Button" HorizontalAlignment="Left" Margin="374,34,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/>
    </Grid>