We go to the beach yearly, and have shared a house with friends before. I know a lot of planning went into the trip prior to leaving as to money and food. Even if your friend usually goes for free, she has to have some concept of how much, even 2 days, costs for lodging and food. No matter what day they come, you need to simply say that you cannot accomodate their meals and snacks. The hardest part could be the children, and if they are taking drinks or food without asking, I would make sure to speak up and ask that they replace these things (not give you money, because then you have to shop) before they leave for their house.
In the future, I would make sure not to quickly agree to a friend and instead, if something like this comes up, respond either no, or say you have to talk it over with your husband, so that at least you can really think about the consequences of your decision. It can be easy to say, oh sure, but then when you get home and realize what you've agreed to, you want to kick yourself.