You have to write javascript function within the check_box_tag in order
to verify whether the checkbox is checked or not and then enable the
hidden field if it is checked.
when field is disabled, it's not taken into account when form is sent
PS: if not using jQuery, it's simple to do with DOM too, just add :onClick='disableField()' to the checkbox and
function disableField() {
document.getElementById('hidden_field').disabled = !document.getElementById('my_checkbox').checked;
return false;
}