Also, if they have not planned for appetizers ready when guests arrive, they are ESSENTIAL with this many kids. Hot hungry kids who have to wait the hour for the post photography couple to arrive spell disaster for the dinner. Since it is a hot barn, I suggest:
cold watermelon in slices or chunks- star with about 60 pounds
www.delish.com/cooking/a21973640/how-to-cut-watermelon/
tiphero.com/quick-and-easy-watermelon-cut

cheese logs and spreads- gingersnaps or lemon snaps for part of the crackers 8 pounds cheeses, 8 pounds crackers and cookies

some kind of dip such as hummus or ranch or spinach dip with finger veggies

pulled pork, about 20 pounds, 100 slider rolls
bbq chicken, 80 pounds leg quarters, cut some no two pieces after cooking. Strongly suggest these are cooked ahead, chilled and reheated, rather than grilled on site, or you will need 2 people just for grilling
Kielbasa sausage, if whole, about 40, dozen buns. If cuts, about 12 pounds
Suggest, 1 crock pot full of vegetarian sausages, I like Field Roast brand, for any non-meat eaters
onions- 8 pounds sliced and soaked
pickles- 1 gallon
slaw- 2-3 gallons
Have bbq sauce on tables

green beans, 24 pounds
baked beans, 5 #10 cans or 10 pounds dry. Make meatless, or have at least 1 pan labeled meatless
corn dish, Start w 22 pounds of corn, additions per your recipe

macaroni, 5 pounds dry pasta
potato, 30 pounds potatoes
green or tossed, use plan for 100 able, do for 100. Strongly consider putting serving bowls of salad plu dressings and rolls on tables so people can eat and visit while waitng for buffet turn.

lemonade, 6 gallons, frozen much better than powdered
unsweetened tea, 6 gallons plus sweeteners

rolls/bread 15 pounds total, count the rolls

water- pint per person, serving in large clear dispensers with cucumber/mint and separate citrus infusions is very popular right now
ice- half pound per person direct plus as needed for icing watermelons and food