It's not a burden. I want to do this for them.
The zoo encourages people to bring coolers and has a picnic area set up. We will have the baby's wagon. The big cooler with the main lunch stuff can stay in the car. And a smaller cooler with drinks and snacks will fit in the wagon with baby.
Okay, that changes things up a bit!
For lunch, I like jedikaiti's suggestion of sandwich stuff.
I would go with sandwich rolls instead of loaf-bread, just to dress it up a bit. Kaiser, pretzel, bagel, etc. For 10 people? I'm thinking 15 rolls. The rolls are more filling, so you'll probably go home with extras. But just in case someone wants seconds, better safe than sorry.
Instead of buying meats from the deli, you can bake up some chicken breast and/or a roast (beef, pork or both, depending on the variety you're looking for) then slice it thinly for sandwiches. Whatever you don't bring for the picnic can always be frozen for later use.
A couple of different cheeses, chopped lettuce, sliced tomato (in a hard container so they don't get mushy), mayo, mustard and you're good!
You could also pack a couple of different salads . . . potato, cole slaw, bean . . . you know these kids better than I do. Do they have any preferences? I think one or possibly two choices would suffice (depending on how much room you have in your cooler.)
Or, instead of a salad, how about a veggie assortment? . . . Crudités! (I learned a new word.
. . . easily packed in baggies) and a bottle of ranch dressing.
Dessert suggestion -- pack a can of whip cream (the spray kind like Redi-Whip) to serve on a fresh-chopped fruit mixture.
Here's a suggestion (doesn't fit in with the above menu, but maybe good for a late hardy snack for the drive home when kids are hungry and looking for dinner, or something that might fit with a different menu of your choosing . . . just throwing this out there for thought, kids love them!):http://www.pillsbury.com/recipes/piglets-in-the-blanket/ae4c1349-34be-4c89-a8a1-a166cf1da49b
I use mini-cocktail-sized smokey links instead of hot dots. They really are good cold.
I suggest the smaller water bottles (6-8 oz size) Sometimes someone (especially kids) only wants a sip or two of something to drink.
Bring a marker so that you can write a name on the bottle then toss it back into the cooler if necessary.
There's quite a variety of single-serving mixes of flavorings that can easily be added to water.
Snacks: This is kind of tough, because all kids don't get hungry for a snack at the same time.
Do you plan on an "It's Snack Time" type of deal where everyone takes a break for snacking? or would you like to have something available whenever someone says "I'm huuuuungry!"
If it's the latter, you can have larger bags of chips/popcorn/trail mix, maybe a bag of cubed cheese, chunks of sausage and crackers . . . whatever your preference, then bring a box of sandwich sized baggies so that you can dole out smaller single-serving sizes.
Of course you can always fill your cooler with fresh fruits (apples, oranges, bananas)
Just me thinking out loud. Hope this helps.