I think I'd do a lot of "assemble your own" X: tacos, sandwiches, single serve pizza, salad bar. That way you can satisfy the lactose-intolerant meat-eater as well as the vegan. And also yourself and your kids.

Here are some options:

Fried zucchini pasta - fry slices of zucchini in olive oil and season with salt and pepper and toss with spaghetti

Minestrone soup made with vegetable stock - if you add beans and pasta its a whole meal

Fruit crisp made with vegan butter

Ratatouille (without onions) over rice or pasta

Cook in components, so that everyone can eat something, even if people cant eat everything. So, a roast something, a pot of beans (vegetarian/vegan), a starch, a vegetable, and a salad.

While it sounds good, how is Mediterranean not "ethnic flavoring"? Falafel, couscous, hummus, olives and feta are all acquired tastes and I know in our group of friends of pretty liberal eaters that at least one person wouldn't eat each of these. These guests do not sound "foody" and that meal sounds pretty "foody" to me.

Some suggestions:
- Pasta primavera (substitute bread crumbs and olive oil for the parmesan) or ratatouille without onions. You can select the veggies that are in season and look good at the market
- Vegetarian paella http://www.foodnetwork.com/recipes/robin-miller/vegetarian-paella-recipe/index.html or Vegetarian Jambalaya http://vegetarian.about.com/od/maindishentreerecipes/r/Jambalaya.htm
- Pasta with a nice bolognese sauce (just kidding), with olive oil, tomatoes, basil, and roasted red pepper. You can make breaded medallions of chicken, eggplant, tofu, whatever on the side that you pan fry in butter or olive oil and italian seasonings that people can add on top of the pasta per their choice.
-various hearty soups that can be made and served with corn bread and salad. For those who want a heartier meal with meat, a beef stew would work here. Split pea or roasted tomato and red pepper for the vegans, etc.

I frequently make meals where you have one base and then people add their protein of choice to the top. That often satisfies people with disparate dietary restrictions.

Do pasta with salad - you can add eggplant, brocolli or other vegetables if you think it needs to be heartier.

This is OP - thanks! Mediterranean is a good idea. Falafel patties, salad, couscous, hummus, olives, feta for the cheese eaters, pita. Thanks! They'd probably like that, and hummus is big in our house.

Hummus is a big one, or roasted vegetables. You can also make or serve vegetarian chili. As a person with food restrictions, I generally bring my own food when I visit so as not to disrupt whatever the host plans to serve.

Family is coming this weekend and among them they are: vegan, lactose-intolerant, onion-intolerant, and do not care much for ethnic flavors.

I am a food person, and yet the only thing I can think of is tacos: a variety of shells, boca "meat," veggie toppings, cheese for those who eat cheese, salsa (sorry, can't get around the onions here), guac (they do like guac), some type of onion-less rice dish, beans, and beverages.

Any other ideas? I probably only need a lunch and a dinner since it's just one weekend and we'll eat out.