this.B = new ObservableCollection<B>();
 this.A.ToList().ForEach(a => a.B.ToList().ForEach(b => this.B.Add(b)));

 <DataGrid ItemsSource="{Binding B}" AutoGenerateColumns="False">
     <DataGrid.Columns>
         <DataGridTextColumn Width="*" IsReadOnly="True" Header="Data" Binding="{Binding Data}"/>
     </DataGrid.Columns>
 </DataGrid>