Grills are an ideal vector for cross-contamination because food is generally placed directly onto grill grates and moved with one spatula or set of tongs. Ideally, grill owners clean their grates thoroughly after every use, but not everyone meets this ideal -- especially if the grill in question is at a campsite or park.
Beware the Marinade
In addition to the possibility of cross-contamination, marinades and sauces often include unexpected common allergens such as nuts, corn, soy, wheat, eggs, onions, sesame, and even dairy. Seasonings can be mixed into burgers or brushed lightly onto foods, and it's not always possible to tell by looking at a grilled food what allergens might be present. Even when grillers are aware of cross-contamination, or all the foods appear to be safe, ask about (and remind children to ask about) ingredients. Whenever possible, go to your outdoor event a little early and make yourself available to read labels on commercial marinades.
What You Can Do
There are three very easy ways to enjoy allergy-safe grilling without much hassle. Another is possible, but requires some meticulous preparation. All are aimed at avoiding the possibility of contact between allergens and safe food.
First, you can cook safe foods on a community grill using foil packs or a cast-iron skillet. No, you won't get charring or grill marks using either of these methods. But your food won't touch the grill and should avoid any allergens that may have been on the grill as the result of previous use. For utmost safety, use multiple layers of heavy-duty foil to make a foil pack (to protect against rips or tears in any one layer) and, of course, thoroughly clean your cast iron skillet before using it again. Be sure, too, to avoid cross-contamination between grilling tools and the food in the skillet, and to avoid touching the part of the foil pack that was in contact with the grill (for children, parents will probably want to remove the food themselves and let it cool).
Second, if you're among the families for whom grilling tends to be a several-times-a-week affair, you might consider investing in an inexpensive portable grill and a separate set of grilling tools to be kept strictly allergen-free. This solution isn't economical for everyone, but it may be worth considering if you grill frequently.
Another easy but expensive option, if you have a gas grill, is to get an extra set of grates and keep them allergen-free.
The meticulous solution is to clean a shared grill very thoroughly and carefully, and to cook the allergen-free food on the grill before any foods that include allergens.
No matter which option you use, don't overlook grilling tools as a potential source of cross-contamination. Spatulas, tongs and brushes are all potential vectors of cross-contamination. And be sure to plate allergen-free food onto clean plates. With care and caution, you can enjoy summertime grilling safely.