Changing Textbox text on checking checkbox in gridview

I have created a grid view in which there are certain number of rows of text box and checkbox. Now i want on checking the checkbox the text of textbox should be changed from 'A' to 'P'. This is my design View.

<asp:GridView ID="grdData" runat="server" style="Text-align:center;">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:CheckBox ID="CheckBox1" runat="server" OnCheckedChanged="grdData_SelectedIndexChanged"/>
</ItemTemplate>
<HeaderTemplate>
<asp:CheckBox ID="CheckBox2" runat="server" OnClick="CheckAllEmp(this)"/>
</HeaderTemplate>
</asp:TemplateField>
<asp:TemplateField>
<HeaderTemplate>
<asp:Label ID="Status_Header" runat="server" Text="Status" />
</HeaderTemplate>
<ItemTemplate>
<asp:TextBox ID="TextBox1" runat="server" Enabled="false" Text="a"></asp:TextBox>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

This is my Code Behind

protected void chkview_CheckedChanged(object sender, EventArgs e)
{
for (int i = 0; i < grdData.Rows.Count; i++){
CheckBox ck =(CheckBox)grdData.Rows[i].Cells[0].FindControl("CheckBox1");
Label Child_Label =(Label)grdData.Rows[i].Cells[5].FindControl("Child_Label");

if (ck.Checked == true){
Child_Label.Text = "Present";}
else
Child_Label.Text = "Absent";}}