I had to change my hours because of one family as well. I was originally open 7-5:30 but found that one family was arriving right at closing dad had a change of clothes on, Mom as well, would arrive in her gym clothes. My older children have activites 3 evenings a week and we must leave the house by 6:15 so it is a real time crunch. I was getting so annoyed with this one family arriving right at closing knowing that they had gone home first or were running errands daily. All my other kiddos are picked up by 4:45! This was almost an hour later!
I ended up sending this one family a notice letter which I addressed to "Dear Parents" even though I only gave it to them giving 6 weeks notice of my time change to 5:00 pm. They have since been able to accomodate but the odd time Dad is maybe 5 mins late and always apologizes profusely when he is so I don't mind.