There isn't anything in the XamDataTree that will do this natively.  You might be able to get some results if you  your item template with two elements, the grid and a textblock that shows your display text

             <ig:NodeLayout Key="layout1" TargetTypeName="Person" DisplayMemberPath="Name" >
                    <ig:NodeLayout.ItemTemplate>
                        <DataTemplate>
                            <StackPanel>
                            <TextBlock Text="{Binding Data.Name}" Visibility="{Binding Converter={StaticResource tbConv}}" ></TextBlock>
                                <ig:XamGrid ItemsSource="{Binding Data.SubNames}" Visibility="{Binding Converter={StaticResource gridConv}}" ></ig:XamGrid>
                            </StackPanel>
                        </DataTemplate>
                    </ig:NodeLayout.ItemTemplate>
                </ig:NodeLayout>

Then have ValueConverters on the visibility property of the two objects and turn them on and off depending on if you have children nodes or not.

    public class TextblockConverter : IValueConverter
    {
        #region IValueConverter Members

        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            XamDataTreeNodeDataContext context = value as XamDataTreeNodeDataContext;
            if (context != null)
            {
                Person p = context.Data as Person;

                if (p != null)
                {
                    return p.Children != null && p.Children.Count > 0 ? Visibility.Visible : Visibility.Collapsed ;
                }
            }
            return Visibility.Visible;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }

        #endregion
    }