web user control not calling from aspx page, kept as a child element

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C#

ASP.NET

Javascript

C#4.0

Hi All,
I had created a web user control for drop down checked box list, following is code

<asp:TextBox ID="txtCombo" runat="server" ReadOnly="true" CssClass="DropDownCss" BackColor="#EEEEEE" Width="225" Height="25" ></asp:TextBox>
<cc1:PopupControlExtender ID="PopupControlExtender111"  runat="server" 
    TargetControlID="txtCombo" PopupControlID="Panel111" Position="Bottom" >
</cc1:PopupControlExtender>

<input type="hidden" name="hidVal" id="hidVal" runat="server" />

<asp:Panel ID="Panel111" runat="server"  Width="230" BackColor="#EEEEEE" BorderColor="Gray" BorderWidth="1">
    
    <asp:CheckBoxList ID="chkList" 
        runat="server" 
        Height="150" onclick="CheckItem(this)"         
        >                                                                                                                                                                        
    </asp:CheckBoxList>
 
</asp:Panel>

In this I am calling a javascript function to get current selected list item.

function CheckItem(checkBoxList) {
       //code here
       
    }

when I am keeping this control is a chile element on aspx page, it is failing to load the javascript function.
In browser showing message as "Uncaught ReferenceError: CheckItem is not defined"

If control is having only one parent control then working fine.

How to overcome this issue.

Thanks in advance.

-Avinash