How to specify foreign keys between two DataTables in SQL Bulk Copy operation?

I have two DataTables A and B. A is the parent table. Rows in B contain a field ParentId that references A.

I want to bulk insert A and B separately. After inserting A, how do I set the ParentId in the corresponding child rows in B?

Update:

I have the following code for creating the DataTables:

    var a= new DataTable("A");

    DataColumn id= new DataColumn("Id", typeof(int));
    billablePriceMapId.AutoIncrement = true;
    billable.Columns.Add(id);

    DataColumn fee = new DataColumn("Fee", typeof(decimal));
    billable.Columns.Add(fee);

    DataColumn[] keys = new DataColumn[1];
    keys[0] = id;
    billable.PrimaryKey = keys;

For the child table:

    var b= new DataTable("B");

    DataColumn id= new DataColumn("Id", typeof(int));
    billablePriceMapId.AutoIncrement = true;
    billable.Columns.Add(id);

    DataColumn parentId= new DataColumn("ParentId", typeof(int));
    billable.Columns.Add(parentId);

    DataColumn[] keys = new DataColumn[1];
    keys[0] = id;
    billable.PrimaryKey = keys;

I have not set up any relationship between the two tables. I would like to know how I could do that the right way.

Following is the code that I use for bulk insert:

using (SqlBulkCopy bulkCopy = new SqlBulkCopy(connection, SqlBulkCopyOptions.Default, transaction))
                    {
                        try
                        {
                            bulkCopy.DestinationTableName = "dbo.A";
                            bulkCopy.WriteToServer(a);

                            bulkCopy.DestinationTableName = "dbo.B";
                            bulkCopy.WriteToServer(b);
                        }
                        catch (Exception ex)
                        {
                            Logger.Error(ex, "Bulk copy operation failed.");
                        }
                    }