I'm writing a small test app to try and figure this out and so far I have this :

  <igDP:XamDataGrid
    x:Name="xamDataGrid"
    DataSource="{Binding}"
      Theme="Office2k7Blue"
    >

        <igDP:XamDataGrid.FieldLayoutSettings>
            <igDP:FieldLayoutSettings
                AutoGenerateFields="False"
                ExpansionIndicatorDisplayMode="CheckOnDisplay"
                HighlightAlternateRecords="True">
            </igDP:FieldLayoutSettings>
        </igDP:XamDataGrid.FieldLayoutSettings>

        <igDP:XamDataGrid.FieldLayouts>
      <igDP:FieldLayout Key="master">
        <igDP:FieldLayout.Fields>
            <igDP:Field Name="SubLocations" IsExpandable="True"/>
            <igDP:Field Name="ID" Visibility="Collapsed" />
            <igDP:Field Name="Name" />
            <igDP:Field Name="Description">
          </igDP:Field>
        </igDP:FieldLayout.Fields>
      </igDP:FieldLayout>

      <igDP:FieldLayout Key="detail">
        <igDP:FieldLayout.Fields>
            <igDP:Field Name="ID" />
            <igDP:Field Name="Name" />
            <igDP:Field Name="Description" Width="20" />
        </igDP:FieldLayout.Fields>
      </igDP:FieldLayout>
    </igDP:XamDataGrid.FieldLayouts>

  </igDP:XamDataGrid>

Which is bound to some data using this class hierarchy :

    public class LocationParent : Location
    {
        public int NumChildren { get { return SubLocations != null? SubLocations.Length : 0; }}
        public Location[] SubLocations { get; set; }
    }

    public class Location
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public string Description { get; set; }       
    }

Everywhere I've read so far suggests that adding IsExpandable="True" to a field will indicate to the data grid that that field's children should have a field layout applied to them, but this doesn't happen and all I end up with is "Location[] Array" displayed when I expand the item.

This may be a misunderstanding on my part, but what I also don't understand is why, when I have AutogenerateFields="False", the records are not expandable at all.

I'd like to be able to show child records which have a field layout showing only some of their actual properties, but unless AutoGenerateFields is set to true, I can't seem to ever show them, and when that's the case, the datagrid auto generates all the fields I don't want to see!