This may sound unnecessary with only a few employees who all have direct access to the boss, however we do think it is good practice to provide an anonymous way for a concern to be raised – even though in practice the best way to address it would most likely be to get everyone together anyway!
There is something useful also, as a minimum, in rating how happy your employees are in their workplace rating 1-5 and measure against prior years (or every 2 years which is what we ask). A Survey Monkey survey with 3 or 4 questions would be sufficient to meet this requirement.