XML data is currently not 100% supported by the grid. That said, you could refer to this thread to get an idea of how to proceed.

As for display the values of 2 subnodes in a cell, I suggest you create a new DataTemplate that consists of a Panel and 2 TextBlocks whose Text properties are bound to the n and v nodes. This template should then be provided to the column's CellContentTemplate property. Something like:

<xcdg:DataGridControl.Columns>
   <xcdg:Column FieldName="p">
      <xcdg:Column.CellContentTemplate>
         <DataTemplate>
            <StackPanel>
               <TextBlock Text="{Binding Path=n}"/>
               <TextBlock Text="{Binding Path=p}"/>
            </StackPanel>
         </DataTemplate>
      </xcdg:Column.CellContentTemplate>
   </xcdg:Column>
</xcdg:DataGridControl.Columns>

Of course this assumes that you implemented the details in the first thread regarding XmlDataGridItemProperty objects.