Please call the 'getParentRowIndex' method to get the index of parent row.
onExpandRow: function(index,row){
var ddv = $(this).datagrid('getRowDetail',index).find('table.ddv');
ddv.datagrid({
onClickCell: function(){
var parentDg = $(this).datagrid('getParentGrid');
var parentIndex = $(this).datagrid('getParentRowIndex');
}
}
}