Dynamically created graphical checkboxes are not found in panel.controls list

I am creating dynamic graphical checkboxes based on my database values inside a panel. but when i try to change the checked state while clicking on the select All checkbox. Nothing happens. I cannot find any controls inside that.

<div style="overflow: auto; height: 462px; width: 386px; margin-top: 0px;">
                            <asp:Panel ID="optionPanel" runat="server" Height="479px" Width="380px">
                                <cc1:GraphicalCheckBox ID="chkSelectAll" runat="server" CheckedImg="images/checked.gif" OnCheckedChanged="chkSelectAll_CheckedChanged"
                                    UncheckedImg="images/unchecked.gif" UncheckedOverImg="images/unchecked.gif" CheckedOverImg="images/checked.gif"
                                    UncheckedDisImg="images/unchecked_disabled.gif" Style="cursor: hand;" Font-Size="100%" AutoPostBack="true"/>
                                <span class="survey_wizard_title" id="lblSelectAll" runat="server">Select All</span>
                            </asp:Panel>
                        </div>

`

foreach (OptionPool opt in respOptions)
    {
        Panel row = new Panel();
        GraphicalCheckBox cb = new GraphicalCheckBox();
        row.Attributes.Add("class", "surveyCheckbox");
        cb.ID = Convert.ToString(opt.id);
        cb.Attributes.Add("value", opt.id.ToString());
        cb.Text = opt.data;
        cb.Style.Add("cursor", "hand");
        cb.Font.Size = new FontUnit("100%");
        cb.UncheckedImg = "images/unchecked.gif";
        cb.UncheckedOverImg = "images/unchecked.gif";
        cb.CheckedImg = "images/checked.gif";
        cb.CheckedOverImg = "images/checked.gif";
        cb.UncheckedDisImg = "images/unchecked_disabled.gif";

        row.Controls.Add(cb);
        optionPanel.Controls.Add(row);
    }


 protected void chkSelectAll_CheckedChanged(object sender, EventArgs e)
{
    if (chkSelectAll.Checked == true)
    {
        foreach (Control child in optionPanel.Controls)
        {
            if (!(child is GraphicalCheckBox))
                continue;

            if ((child is GraphicalCheckBox))
            {
                (child as GraphicalCheckBox).Checked = true;
            }
        }
    }
}

optionPanel.control doesn't list any of the above dynamically created checkboxes.