I have a Custom object that represents inventory items and looks similar to this:

 public class InvItem {

        public string ItemID {
            get;
            set;
        }

  public string UPC_SKU {
            get;
            set;
   }
   public string StockingUM {
            get;
            set;
   }
public decimal Price {
            get;
            set;
 }
 public decimal Weight {
            get;
            set;
 }
public ObservableCollection<InvCategory> AttachedCategories {

            get { return _categories; }
            set { _categories = value; }
}
public ObservableCollection<InvDocument> Documents {
            get { return _documents; }
            set { _documents = value; }
}
public ObservableCollection<string> Tags {
            get { return _tags; }
            set { _tags = value; }
}
 
public InvItem() { }

}

The InvItem class is stored in an ObservableCollection<InvItem> 
which is populated with a call to my database. I then bind the 
collection to my XamDataGrid. Allowing the XamDataGrid to 
AutoGenerate Fields and Layout, it discovers the Hierachical 
data structure between ObservableCollection<InvItem>  
and it's child collections, <AttachedCategories>, 
<Documents>, etc, 

But, I am trying to build a manual field layout very similar 
to the 'Record Layout Customization' sample in the Feature 
Browser. For the child collections, I would like to use a 
ComboBox or XamComboEditor, both of which I have 
been able to embed in the XamDataGrid, but I have been 
unable to bind either to the child  collections. 

Any help would be greatly appreciated.
Rob