Is it possible to activate customfield ‘text description’ upon activating the customfield ‘checkbox’?

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>