For my daughter's first we did a garden party theme - which could work well for a boy/girl party too - flowers and bugs!! Look at the invites at practicallydarlingdesigns.com for ideas and/or wording. For an activity, I bought some small clay pots and had the kiddies paint them with the party themed colors. Then I cut out some flower head shapes out of construction paper - 1 flower per 8 1/2 x 11 so pretty big, then cut an X through the middle and then push a regular size flashlight through the whole so it makes the "stem" of the flower, and secure it in the pot with some moss from the craft store. I bought the flashlights at Target and they were like 2 for $3 and came with batteries. All children LOVE flashlights and that way you don't have to get treat bags with a bunch of junkie toys that will break in 2 minutes, or candy that is no good for them. You could also have them make bugs with like styrofoam shapes or cut egg cartons, paint, pipe cleaners, and googly eyes. (Just make sure you have some spare t-shirts for the kids to wear while doing crafts, so the parents don't get mad at you for ruining their clothes.)
As for food, I think you can go with simple like hot dogs and hamburgers, as long as you have some special side dishes - like Jello jigglers cut into flower/bug shapes with cookie cutters, or fruit shishkabobs (although a huge pain to make - pineapple, red grape, orange slice, green grape, pineapple - the kids and grown ups LOVE them.) I'd also suggest making the twins their own little cakes to eat, and then make cupcakes for the everyone else. You should be able to find some plastic bug/flower decorations at a craft store, Walmart, or even the dollar store. I know that Bettycrocker.com also has some ways to decorate cupcakes to look like bugs, but who has that kind of time? I guess you could do an example cupcake and let the kids make their own bugs on top though?
Okay - can you tell I love to plan parties? To get more ideas, just do a yahoo search on party ideas and you'll get a ton of sites to peruse. Good luck and have fun with this - they only turn one once!