My business close same time, every day, all year. Some client have found hard in Winter but must consider when coming here. I charge for late collect and if it on-going issue I will terminate care. They have arrange to leave on time, find daycare open later or have someone able to pick up if they held up.
Be harda$$. If they went grocery shopping after closing time, guess what? Store closed. Just because we can't place child on step and leave, that not mean we have to tolerate.
As you say, if you were centre, there would be zero tolerance.
If their child in Brownie or Scout, they manage get to Community Hall on time and end of session. It their responsibility as parent to be there pick up child when activities end, when friend's party ends, when school concert ends, when playdate ends. This no different.
I have had family in past who come very last minute even though they finish early or even if off for day. Once their financial penalty for these type parent, they will usually be on time. And if not, be firm that they might have find alternative care if it continue.