The following example demonstrates how to handle the PreviewMouseLeftButtonDown event on the GroupHeaderControl objects contained in the headers of the child groups to toggle the expansion state of child groups using the ToggleGroupExpansion method. The group whose state is to be toggled will be retrieved using the GetParentGroupFromItem method.

XAML
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<Grid xmlns:xcdg="http://schemas.xceed.com/wpf/xaml/datagrid">
  <Grid.Resources>
    <xcdg:DataGridCollectionViewSource x:Key="cvs_orders"
                                       Source="{Binding Source={x:Static Application.Current},
                                                        Path=Orders}">
      <xcdg:DataGridCollectionViewSource.GroupDescriptions>
        <xcdg:DataGridGroupDescription PropertyName="ShipCountry"/>
        <xcdg:DataGridGroupDescription PropertyName="ShipCity"/>
      </xcdg:DataGridCollectionViewSource.GroupDescriptions>
    </xcdg:DataGridCollectionViewSource>
    <Style TargetType="{x:Type xcdg:GroupHeaderControl}">
      <EventSetter Event="PreviewMouseLeftButtonDown"
                   Handler="HeaderDown"/>
    </Style>  
</Grid.Resources>
  <xcdg:DataGridControl x:Name="OrdersGrid"
                        ItemsSource="{Binding Source={StaticResource cvs_orders}}"/>
</Grid>

The following code provides the implementation of the PreviewMouseLeftButtonDown event in which we will retrieve the item represented by the GroupHeaderControl (GroupHeaderFooterItem) using the GetItemFromContainer method, which will then be used to retrieve the parent group (GetParentGroupFromItem) whose state is to be toggled.

VB.NET
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Private Sub HeaderDown( ByVal sender As Object, ByVal e As MouseButtonEventArgs )
  Dim headerControl As GroupHeaderControl = TryCast( sender, GroupHeaderControl );
  If headerControl Is Nothing Then
    Return
  End If
  Dim context As DataGridContext = DataGridControl.GetDataGridContext( headerControl )
  Dim item As Object = context.GetItemFromContainer( headerControl ) )
  If Not item Is Nothing Then
    Dim group As CollectionViewGroup = context.GetParentGroupFromItem( item )
    If Not group Is Nothing
      context.ToggleGroupExpansion( group )
    End If
  End If
End Sub
C#
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private void HeaderDown( object sender, MouseEventArgs e )
{
  GroupHeaderControl headerControl = sender as GroupHeaderControl;
  if( headerControl == null )
    return;
  DataGridContext context = DataGridControl.GetDataGridContext( headerControl );
  object item = context.GetItemFromContainer( headerControl );
  if( item != null )
  {
    CollectionViewGroup group = context.GetParentGroupFromItem( item );
    if( group != null )
    {
      context.ToggleGroupExpansion( group );
    }
  }
}