Oh my goodness, I'm sorry because I hate to say this but you are not only being waaay too nice but you are being a doormat that these people are wiping their feet upon. What exactly did your 'written notice' say to them. I would not have any problem giving this family a letter of warning on Monday telling them that you will not tolerate any more late days and that if they are ever late again they are done. Are you advertising to replace them?