I am very firm about drop off times. I'm open at 7:30, no exceptions. I just had an interview go south because the parents told me that it was more convenient for them to drop off at 7:00 and I said no go. I have my own family to take care of before I open my door.
As far as pick up time goes, I have a small amount of give. For example, dcm told me Monday that she might be 15 min late Fri. She gave me lots of notice, and they are never later than they should be. This family has respected all of my policies right from the word go, so they get a bit of leash.
I would speak to them about it. If you don't want this to become a regular thing with them, and you are not ok with adjusting your hours at all - tell them. It sucks having to enforce things, and they may feel that you're being nit picky, but if you don't enforce your own rules, who will? Most parents will take a mile if you give them an inch.