A possible solution is to use jquery to show/hide your custom field, something like this:
<script>
jQuery(document).ready(function($) {
$('#yourcheckboxidentifier').click(function() {
$('#customfieldidentifier').fadeToggle(400);
});
if ($('#yourcheckboxidentifier').val() !== undefined) {
$('#customfieldidentifier').show();
}
});
</script>