Error in C# Code for Parent-child Relationship between tables

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C#2.0

C#

ASP.NET

Below code is used to set Parent-child relationship between 2 tables and display through 2 dataGridViews.

using (SqlConnection con = new SqlConnection("Data Source=.\\SQLEXPRESS; database= NORTHWND; Persist Security Info=false; Integrated Security=true;"))
            {
                SqlCommand cmd1 = new SqlCommand();
                SqlCommand cmd2 = new SqlCommand(); 
                cmd1 = con.CreateCommand();
                cmd2 = con.CreateCommand();
                cmd1.CommandText = "Select * from Customers";
                cmd2.CommandText = "Select * from Orders";
                cmd1.Connection = con;
                cmd2.Connection = con;

                con.Open();
                    SqlDataAdapter adp1 = new SqlDataAdapter(cmd1);
                    DataSet ds = new DataSet();
                    adp1.Fill(ds, "Customers");
                    SqlDataAdapter adp2 = new SqlDataAdapter(cmd2); 
                    adp2.Fill(ds, "Orders");
                DataRelation relation = new DataRelation("CustomersOrders", 
                    ds.Tables["Customers"].Columns["CustomerID"], 
                    ds.Tables["Orders"].Columns["CustomerID"]); 
                    ds.Relations.Add(relation);  // add relation 
                    dataGridView1.DataSource = ds; 
                    dataGridView1.DataMember = "Customers";
                    dataGridView2.DataSource = ds; 
                    dataGridView2.DataMember = "CustomersOrders"; // this line shows error 
            }

Code shows following error message on execution, for the last line of code:
“Childlist for field “CustomerOrders” can’t be created.”
What is the reason and what should be the solution?