If I have to have a chat with a parent, I send them a brief email during the week that says something to the effect of At pianist 1, 2, 3, and 4's next lesson, I would like to have a chat with you about bathroom cleanliness in my studio. Have a great day! Sincerely, That gives the parent a chance to absorb the topic during the week, if she/he is wise, to have a pre-chat with her pianists' about the topic, and to be ready for you when the parent and pianists arrive at your door. Simple, straight to the point, and everyone knows the conversation is coming.
And it gives you a chance to plan out exactly what you are going to say. Maybe you will have 3 rules written down to keep you on track and a cute sign in your bathroom. One of those rules should be: you make a mess, you clean it up.
Before that family leaves, check the bathroom. If it is not to your standard, make them clean it. If the pianists argue and fuss over who made the mess, make a command decision and have one of them clean it. Keep a log of who cleaned the bathroom, so that if it happens again in the future, you can choose a different pianist (or the same one, depending on the situation.) Have cleaner and paper towels standing by if necessary. And be consistent. Every week. If the parent won't enforce the rules at your house (or her own), you have to. Be the Bathroom Na*i.