Not counting the watermelon, for which I always allow 1 pound per person whole, caterers would plan 1 1/2 pounds total food per adult. So allowing 6-8 ounces of meats, you want about a pound total of sides. Not knowing the ages of the kids- I count teens as adults and kids under 12 as 1/2- with around 32 people I would use 1/3 of the amount for 100 as my max for any given dish.

But with this wide selection, it also will help you to visualize- "if everyone took 1/4 cup of this, I would need 8 cups=2 quarts". The few dishes where the average serving is larger, you might add a bit more, but generally, the person who takes a large German potato helping will skip the regular potato salad, so it works out.