Hi DACataldo,

Check the below code.

HTML

<asp:GridView ID="gvCustomers" runat="server" AutoGenerateColumns="false" CssClass="Grid"
    DataKeyNames="CustomerID" OnRowDataBound="OnRowDataBound">
    <Columns>
        <asp:TemplateField HeaderText="Contact Name">
            <ItemTemplate>
                <asp:Label Text='<%#Eval("ContactName") %>' runat="server" />
                <asp:HiddenField ID="hfShowHide" runat="server" Value="Hide" />
                <asp:Panel ID="pnlOrders" runat="server" Style="display: none">
                    <asp:GridView ID="gvOrders" runat="server" AutoGenerateColumns="false" CssClass="ChildGrid">
                        <Columns>
                            <asp:BoundField ItemStyle-Width="150px" DataField="OrderId" HeaderText="Order Id" />
                            <asp:BoundField ItemStyle-Width="150px" DataField="OrderDate" HeaderText="Date" />
                        </Columns>
                    </asp:GridView>
                </asp:Panel>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:BoundField ItemStyle-Width="150px" DataField="City" HeaderText="City" />
    </Columns>
</asp:GridView>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    $("[id*=gvCustomers] td").live("click", function () {
        if ($(this).closest("tr").find('[id$=hfShowHide]').val() == "Hide") {
            $(this).closest("tr").after("<tr><td colspan = '999'>" + $(this).closest("tr").find('[id$=pnlOrders]').html() + "</td></tr>")
            $(this).closest("tr").find('[id$=hfShowHide]').val("Show");
        } else {
            $(this).closest("tr").next().remove();
            $(this).closest("tr").find('[id$=hfShowHide]').val("Hide");
        }
    });
</script>

Code

C#

using System.Configuration;
using System.Data;
using System.Data.SqlClient;

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        gvCustomers.DataSource = GetData("select top 10 * from Customers");
        gvCustomers.DataBind();
    }
}

private static DataTable GetData(string query)
{
    string strConnString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
    using (SqlConnection con = new SqlConnection(strConnString))
    {
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = query;
            using (SqlDataAdapter sda = new SqlDataAdapter())
            {
                cmd.Connection = con;
                sda.SelectCommand = cmd;
                using (DataSet ds = new DataSet())
                {
                    DataTable dt = new DataTable();
                    sda.Fill(dt);
                    return dt;
                }
            }
        }
    }
}

protected void OnRowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        string customerId = gvCustomers.DataKeys[e.Row.RowIndex].Value.ToString();
        GridView gvOrders = e.Row.FindControl("gvOrders") as GridView;
        gvOrders.DataSource = GetData(string.Format("select top 3 * from Orders where CustomerId='{0}'", customerId));
        gvOrders.DataBind();
    }
}

VB.Net

Imports System.Configuration
Imports System.Data
Imports System.Data.SqlClient

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    If Not IsPostBack Then
        gvCustomers.DataSource = GetData("select top 10 * from Customers")
        gvCustomers.DataBind()
    End If
End Sub

Private Shared Function GetData(query As String) As DataTable
    Dim strConnString As String = ConfigurationManager.ConnectionStrings("constr").ConnectionString
    Using con As New SqlConnection(strConnString)
        Using cmd As New SqlCommand()
            cmd.CommandText = query
            Using sda As New SqlDataAdapter()
                cmd.Connection = con
                sda.SelectCommand = cmd
                Using ds As New DataSet()
                    Dim dt As New DataTable()
                    sda.Fill(dt)
                    Return dt
                End Using
            End Using
        End Using
    End Using
End Function

Protected Sub OnRowDataBound(sender As Object, e As GridViewRowEventArgs)
    If e.Row.RowType = DataControlRowType.DataRow Then
        Dim customerId As String = gvCustomers.DataKeys(e.Row.RowIndex).Value.ToString()
        Dim gvOrders As GridView = TryCast(e.Row.FindControl("gvOrders"), GridView)
        gvOrders.DataSource = GetData(String.Format("select top 3 * from Orders where CustomerId='{0}'", customerId))
        gvOrders.DataBind()
    End If
End Sub