How to get Id of controls in childGridview inside Parent gridview in javascript function (ID of ="ui_access" )

function SelectAll(CheckBox) {

        var chkAc1 = document.getElementById('ctl00_MainContent_CheckBoxAccess').checked;
            if (document.getElementById('ctl00_MainContent_CheckBoxAccess').checked) {
                var ParentGrid = document.getElementById("<%= gvParentGrid.ClientID %>");
                if (ParentGrid.rows.length > 0) {
                   var chidgrid = document.getElementById("<%= gvParentGrid.gvChildGrid.ClientID %>");
            }
}
}

<asp:GridView ID="gvParentGrid" ShowHeaderWhenEmpty="true" runat="server" DataKeyNames="ui_name" Width="100%"
                   AutoGenerateColumns="false" OnRowDataBound="gvParent_RowDataBound" GridLines="none" ShowHeader="false" >
                   <HeaderStyle Height="0px" HorizontalAlign="Center" Font-Size="5px" VerticalAlign="Middle"></HeaderStyle>
                   <RowStyle CssClass="DG_ItemStyle" HorizontalAlign="left"  VerticalAlign="Middle"/>
                   <AlternatingRowStyle CssClass="DG_ItemStyle" HorizontalAlign="left" VerticalAlign="Middle"/>
                   <Columns>
                      <asp:TemplateField>
                           <ItemTemplate>
                           <tr>
                           <td colspan="100%">
                           <div id="div<%# Eval("ID") %>" style="display: BLOCK; position: relative; left: 15px; overflow: auto;">
                                <asp:GridView ID="gvChildGrid" runat="server" OnRowDataBound="gvChild_RowDataBound" AutoGenerateColumns="false" GridLines="none"  Width="650px" >
                               <HeaderStyle Height="32px" CssClass="DG_ChildHeaderStyle" Font-Bold="true" HorizontalAlign="Center" VerticalAlign="Middle"></HeaderStyle>
                               <RowStyle CssClass="DG_ItemStyle" HorizontalAlign="left"  VerticalAlign="Middle"/>
                               <AlternatingRowStyle CssClass="DG_AlternateItemStyle" HorizontalAlign="left" VerticalAlign="Middle"/>
                               <Columns>
                                   <asp:BoundField DataField="UI_NAME" HeaderText="" HeaderStyle-HorizontalAlign="Left" ItemStyle-Width="20%" />
                                    <asp:TemplateField HeaderText="Access" HeaderStyle-HorizontalAlign="Left" >
                                       <ItemTemplate>
                                           <asp:CheckBox runat="server" ID="ui_access" />
                                       </ItemTemplate>
                                   </asp:TemplateField>