This advice is taken from porting a DevEx Hierarchical Grid from DevForce Classic to DevForce EF.

                Try creating a new property on you customer object called “Orders2”.  This property should be a BindableList<Order>.  Then bind to Orders2 instead of Orders in your AddRelationView line.

Here is the custom property on Customer:

    public BindableList<Order> Orders2 {

      get {

        BindableList<Order> mOrders = new BindableList<Order>();

        mOrders.AddRange(this.Orders);

        return mOrders;

      }

    }

Here is the final code for the nested grid:

      _ordersXGBM.GridView = _customersXGBM.AddRelationView("Order", "Orders2");

      this.gridView2.LayoutChanged();

      _ordersXGBM.GridView.OptionsView.NewItemRowPosition = DevExpress.XtraGrid.Views.Grid.NewItemRowPosition.Bottom;