I hope you do respond and let them know how you feel. I think it's possible people continue to behave in certain ways because we don't let each other know what behavior is acceptable and what is not. I don't believe it is necessary to be unkind, but I think it's important to be clear. I think you have every right to have expectations of behavior in your own home. Personally, I would not have let them invite themselves, you are nicer than I am. However, if that were my situation, I would have asked them to supervise their children cleaning up their mess and would have asked them to leave. I bet they would never invite themselves over again!!!!
Juliet