Видел ещё подобный пример:

private static void ExpandRecursively(ItemsControl itemsControl, bool expand)
{
    ItemContainerGenerator itemContainerGenerator = itemsControl.ItemContainerGenerator;
    for (int i = itemsControl.Items.Count - 1; i >= 0; --i)
    {
    	ItemsControl childControl = itemContainerGenerator.ContainerFromIndex(i) as ItemsControl;
    	if (childControl != null) ExpandRecursively(childControl, expand);
    }
    TreeViewItem treeViewItem = itemsControl as TreeViewItem;
    if (treeViewItem != null)
    treeViewItem.IsExpanded = expand;
}