Hierarchical datagrid - refer to child when empty

I have a master/detail editable datagridgrid.
To add edit controls in child grid, I take a map to them in render event.

void Render(DataGridRenderEventArgs<StTest> args)
        var test = args.Grid.Data.FirstOrDefault();
        if (test != null && !childGrids.ContainsKey(test.StSectionId))
            childGrids.Add(test.StSectionId, args.Grid);

This works fine.

The problem is when child grid is empty, since I need to have and "Add new record" button.
Empty children, not having data, are not mapped.

I wonder if there is a way to refer to parent data or pass a parameter to child grid, useful for mapping it