Maya, this dynamic grid does not go easy on me ..
Here the problem :
I modified sample as following in order to have edited values on screen:  
in Page_Load I’ve stored DS into session
// store original dataset in session  
       
       if (!IsPostBack)
       {
…......................................   
           // store original dataset in session  
           Session.Remove("LR2MasterDS");
           Session["LR2MasterDS"] = MasterDS;
           Session.Remove("LR2DetailsDS");

           Session["LR2DetailsDS"] = DetailsDS;
           whdg.DataSource = MasterDS;
           whdg.DataBind();
       }
       else
           whdg.DataSource = (ExhibitLR2MasterDS)Session["LR2MasterDS"];

in the whdg_RowIslandsPopulating  - get the datasource from session :

ExhibitLR2DetalsDS DetailsDS = (ExhibitLR2DetalsDS)Session["LR2DetailsDS"];
ExhibitLR2DetalsDS.type3tblDataTable islandType3Tbl = DetailsDS.type3tbl;
when it gets bind It uses datasource updated in the new added RowUpdating
IslandGrid.DataSource = islandType3Tbl;
IslandGrid.DataBind();

protected void whdg_RowUpdating(object sender, RowUpdatingEventArgs e)
    {

       GridRecord gr = e.Row;
       ContainerGrid g = (ContainerGrid)sender;
       ControlDataField a = g.FindColumn("idLR5Detail");
       if (a != null)
       {
           
           int idMaster = (int) e.Values["idMaster"];
           int idRec    = (int) e.Values["idRec"];
           int newAmnt =  (int) e.Values["Amount"];
           int oldAmnt = (int)e.OldValues["Amount"];
           int nType  = (int) e.Values["Ntype"];
           int dfrnce = (newAmnt - oldAmnt) * ((nType == 1) ? 1 : -1);

           ExhibitLR2DetalsDS DetailsDS = (ExhibitLR2DetalsDS)Session["LR2DetailsDS"];
           ExhibitLR2MasterDS MasterDS = (ExhibitLR2MasterDS)Session["LR2MasterDS"];

            ExhibitLR2DetalsDS.type3tblRow    dr =  DetailsDS.type3tbl.FindByidRec(idRec);
            dr.Amount = newAmnt;
            ExhibitLR2MasterDS.MasterTableRow mr = MasterDS.MasterTable.FindByidMaster(idMaster);
            mr.Amount = mr.Amount + dfrnce;
            Session["LR2DetailsDS"] = DetailsDS;
            Session["LR2MasterDS"] = MasterDS;

            whdg.DataSource = MasterDS;
            whdg.DataBind();
       }
       else
       {
           ExhibitLR2MasterDS MasterDS = (ExhibitLR2MasterDS)Session["LR2MasterDS"];
           int idMaster = (int)e.Values["idMaster"];
           int newAmnt = (int)e.Values["Amount"];
           ExhibitLR2MasterDS.MasterTableRow mr = MasterDS.MasterTable.FindByidMaster(idMaster);
           mr.Amount = newAmnt ;
             Session["LR2MasterDS"] = MasterDS;
           whdg.DataSource = MasterDS;
           whdg.DataBind();
       }

Here is the problem : After row updated it is  posting back and after page_load comes to RowUpdating where Datasources get midfield.  Parent grid get databind and then it goes to
RowIslandsPopulating  where it retrieves the updated DetailsDS from session and does DataBind at the end.  I’m checking in debugger contents of islandGrid.DataSource = islandType3Tbl;  table right before next line ( islandGrid.DataBind(); ) and it is updated table with new values, but in the browser it shows old value. Also the value of the parent grid datasource get modified and it is also reflected in Datasource (MasterDS.MasterDStbl) but old value still shows up.  Is it get cashed somewhere ?  How I can turn it off ?

When the same done to parent grid it goes through the same path, just different branch of RowUpdating and value in browser is new, means it works properly....

Little add on on previous post - If custom summary substituted by SummaryType.Sum;  then it works fine, so the real question is where to attach CalculateCustomSummary after editing starts.

I still did not reach an end. I’m adding custom summary to the child (dynamic) grid and having problem that summary get recalculated, on editing.....

The core of the problem is that child grid created dynamically during RowIslandPopulating event triggered apparently by clicking expansion sign as well as CalculateCustomSummary get initialized at the same token.  When dynamic child grid get opened already and some value get changed it posting back but RowIslandPopulating not getting called anymore and CalculateCustomSummary consequently did not get initialized and blowing up.

It is blowing up the same way if I edit cell and will try to close expanded child grid.

So only one child grid can be open at any given moment and no editing involved then it works

So, if expanded child grid getting rendered from the view state how I would insert (register)
CalculateCustomSummary  in this case ? Or any other approach to this problem ?

I attached small test we site that manifest this problem.

Thanks