you can write a function to manipulate textbox accordingly checkbox inside grid and call that function in client side pageLoad function.
function pageLoad()
{
EnableDisabletextBox();
}
function EnableDisabletextBox()
{
try {
$("input[type=checkbox][id*=chkChild]").each(function () {
if (this.checked) {
$(this).closest("tr").find("input[type=text][id*=txtRemark]").attr("disabled", true);
$(this).closest("tr").find("input[type=text][id*=txtRemark]").val("NA");
$(this).closest("tr").find("input[type=text][id*=txtRemark]").focus();
} else {
$(this).closest("tr").find("input[type=text][id*=txtRemark]").attr("disabled", false);
$(this).closest("tr").find("input[type=text][id*=txtRemark]").val("");
$(this).closest("tr").find("input[type=text][id*=txtRemark]").focus();
}
});
} catch (e) {
alert(e);
}
}