Do you charge late pick-up fees? If they're already mentioned in your contract, ensure you follow through with that, it might be a wake up call for the late pick ups. Also, is it in your contract that parents need to provide wipes, diapers etc?
Unfortunately some people just seem to find it all too easy to take advantage of others without a second thought, but that's why it's vital to stand up for yourself, your standards, and your business in a professional yet 'stern' manner (perhaps stern is not the appropriate word, but I'm tired and not thinking as clearly!) You need to show them that you expect them to stick to their signed contract and everything in it, while being respectful towards you at the same time just as they expect from you. Inevitably you will get those who you'll clash with over something or other, but it's important to stick to your guns and not get walked all over. We're not in this business to be doormats.