//Add Columns
flyGrid.Columns.AddRange(new Column[]
    {
      new TreeViewColumn("Nodes");
      new Column("Data");
     });

//create root node
Node rootNode= new TreeViewNode(new object[]{"Root", string.Empty});
//create sub nodes (children)
Node firstNode= new TreeViewNode(new object[]{"First", string.Empty});
Node secondNode= new TreeViewNode(new object[]{"Second", string.Empty});
//add children
rootNode.Items.AddRange(new NodeBase[]{firstNode, secondNode});
//Add to FlyGrid
flyGrid.Rows.Items.Add(rootNode);