Toggle text field depending on checkbox value in drupal 6
You could do something along these lines:

$('#CHKBOX').click( function() {

  if ($('#CHKBOX').is(':checked')) {
    $('#CHKTEXT').display();
  } else {
    $('#CHKTEXT').hide();
  }

});


where CHKBOX and CHKTEXT are the CSS ids of the form elements in the resulting HTML.  If you know it will be hidden at the start, this can be simplified further into:

$('#CHKBOX').click( function() {
  $('#CHKTEXT').toggle();
});