I can see why there confusion if you charge a day rate and they not getting full day care. I agree that contract has to be clearer and I agree that since you give them your time outside hours without charge, why you think it only fair not to be deducted. I see both sides.

Problem is, your contract not clear that X hours = full day rate so even if closing little early, same rate apply.

I find when I first opened, over time, situation happen where my contract not clear enough to cover and so I amend each time this happen. I think you too should amend contract to say if you need close early, because you not charge for early drop or late pick up, that there expectation for full day fee still. Of course, you will get one family who will always drop early or pick up late once your contract permit this so you have decide if it worth it or if it better to just give slight discount when asked.