Task.Factory.StartNew(() => _model.GetItems(node).Select(n => n))
.ContinueWith(t =>
{
    if (t.Result != null)
    {
        ObservableCollection<ItemValue> children = new ObservableCollection<ItemValue>(t.Result);                                
        int rowIndex = Rows.IndexOf(node);
        node.ChildrenSource = children as INotifyCollectionChanged;
        node.Children.Clear();
        Rows.RemoveAt(rowIndex + 1);
        foreach (object obj in children)
        {
            TreeNode child = new TreeNode(this, obj);
            child.HasChildren = HasChildren(child);
            node.Children.Add(child);
        }
        Rows.InsertRange(rowIndex + 1, node.Children.ToArray());
    }
}, TaskScheduler.FromCurrentSynchronizationContext());

Task.Factory.StartNew(() => _model.GetItems(node).Select(n => n))
.ContinueWith(t =>
{
    if (t.Result != null)
    {
        ObservableCollection<ItemValue> children = _model.GetItems(node);                                
        int rowIndex = Rows.IndexOf(node);
        node.ChildrenSource = children as INotifyCollectionChanged;
        node.Children.Clear();
        Rows.RemoveAt(rowIndex + 1);
        foreach (object obj in children)
        {
            TreeNode child = new TreeNode(this, obj);
            child.HasChildren = HasChildren(child);
            node.Children.Add(child);
        }
        Rows.InsertRange(rowIndex + 1, node.Children.ToArray());
    }
}, TaskScheduler.FromCurrentSynchronizationContext());

ObservableCollection<ItemValue> children = new ObservableCollection<ItemValue>();
Task.Factory.StartNew(() =>
{
    Application.Current.Dispatcher.Invoke(()=> children = new ObservableCollection<ItemValue>(_model.GetItems(node)));                            
})
.ContinueWith(t =>
{
    if (children.Count > 0)
    {                                
        //Тут заполняю контрол
    }
}, TaskScheduler.FromCurrentSynchronizationContext());