Are you planning breakfast, lunch, snack and dinner? or just dinners...?
Keep in mind activities that will making an involved meal more difficult (like are you going to be at the beach through lunchtime and need to brings stuff with you or will you be at the house and cooking).
Where will you be going? Is there any way to find out the stores in the area and the supplies available in the house? (Like is there a grill? Does it need propane or charcoal?)
Some things I can think of off the top of my head are:
-Lasagna, garlic bread, green beans, tossed salad
(can use no bake noodles and disposable pans if there are none big enough)
-Burgers (beef or veggie!), Baked beans, tossed salad
(is there going to be a grill?)
-BBQ Chicken, Cole Slaw, Corn on the Cob, tossed salad
-Lemon Pepper Salmon, potato salad, grilled summer veggies, green salad
-Homemade Pizzas topped with any kind of toppings, tossed salad
-Chicken/Beef/Bean Burritos (assemble your own), Rice & Beans, tossed salad
-leftovers (although don't count on there being any/much with that many people around, it can still be an option for a picky eater if there happens to be some)
-Lots of Sandwiches!- (serve with soups, salad, or veggies & dip)
-grilled cheese w/soup,
-lunch meat (turkey, ham, bologna, salami, cheese, etc.)
-grilled veggie paninis
-PBJ (might want it available as an alternate option for other meals too)
-Taco Dip I know it sounds like a snack but it actually can be a pretty well rounded meal. Mix some heated refried beans with salsa and top with sour cream, shredded cheddar, guacamole (or just diced avocado), and veggies all diced/shredded like tomatoes, lettuce, onions/scallions, peppers, etc. and serve with corn chips. Each person could assemble there own bowl according to preference, or a huge pan of it could be made and everyone scoop out a bowl.
-Pasta salad- whole wheat pasta, bite sized fresh veggies (broccoli, grape tomatoes, carrots, onions, corn, etc.), salad dressing (Italian, balsamic, etc.) and a protein (grilled chicken, beef, salmon/tuna, beans).
pb on graham crackers
s'mores (more of a dessert/special event)
crackers and cheese
veggies & dip
salsa/guacamole & chips
Breakfast (keep in mind some people aren't breakfast eaters, or will just want a cup of coffee/tea)
-French Toast casseroles (the kind you assemble the night before and bake up in the morning)
-Cold Cereal w/Milk
-Yogurt & granola
-Pancakes/Regular French Toast: standing at the stove for the length of time this would require would take it off my list personally, unless people got up and ready in waves and you could just make up all the batter needed for people to cook their own (or their child's) portion as they were ready to eat
-PB on toast (with bananas and a cup of tea! mmmm)
-Fruit Smoothies and muffins (if a blender were available)
-Hot cereals- oatmeal, cream of wheat, farina (I remember this was standard when we'd go on vacation as kids. There were nine from my family and five from my cousins, plus my 2 grandparents and this was the easiest (and probably healthiest) breakfast to make) They can mix in brown sugar, cinnamon, maple syrup, fruit, dried fruit, shredded coconut, diced walnuts, etc. to make it more interesting.
Hope some of these ideas help! Maybe ask the families coming what there favorite meal is?
(don't forget about drinks! Water and LOTS of lemonade/iced tea powder is probably easiest)