// райтер. выполняется в отдельном потоке
        private void writeData()
        {
         ...
             if (needNewParent)
             {
                 currentParentId = appendParent(DateTime.Now);
                 needNewParent = false;
             } 

             writeChild(currentParentId, theSomeData);
         ...
        }

        private Guid appendParent(DateTime aDateTime)
        {
            Guid gd = Guid.NewGuid();
            writeParent(gd, aDateTime);
            return gd;
        }

        private void writeParent(Guid aGuid, DateTime aDateTime)
        }
            lock (datasetLocker)
            {
                theDatasetType.parentRow theParentRow = theDataSet.parents.NewparentRow();
                theParentRow.tube_id = aGuid;
                theParentRow.tube_date = aDateTime;
                theDataSet.parnets.AddparentsRow(theParentRow);
                theDataSet.AcceptChanges();
            }
        }

        private void writeChild(Guid aParentId, Single aSomeData)
        {
            if (aParentId != null)
            {
                lock (datasetLocker)
                {
                    theDatasetType.childsRow theChildRow = theDataSet.childs.NewchildsRow();
                    theChildRow.parent_id = aTubeId;
                    theChildRow.some_data = aSomeData;
                    theDataSet.childs.AddchildsRow(theChildRow); // <== вот здесь получаю исключение по ForeignKeyConstraint
                }
            }
        }