In case you are going to someone else's house for Thanksgiving and are not hosting it like a crazy person, and/or if you have other festive events on the horizon, here are some food allergy hints and tips we've come up with over the years.
For any kind of everybody-bring-a-dish event, bring dessert. Kids are less likely to mind if their veggies don't match everyone else's, but if they are eating a stale granola bar from the bottom of your purse while everyone else feasts on pie, things are going to get ugly. I usually bring something in individual servings, like cookies or cupcakes. That way, there is no fear of the server from a cream pie being used to serve the dairy-free pie. As an added precaution, I put a plate of cookies/cupcakes out for everyone but keep a baggie full of them in my bag. No cross contamination, and this prevents the cookies from being eaten by everyone else first. (This really did happen to us one Thanksgiving, even though I told everyone that they were the dairy-free Katherine cookies and the only dessert she could eat. Fool me once....)
We often bring rolls as well because Katherine loves bread and has a hard time watching everyone else eat it in front of her. Cranberry sauce makes an excellent dairy-free spread. If we don't bring rolls for everyone, I pack a few dairy-free ones just for her.
I also pack a potato. If there are facilities for me to cook it in, I pack it raw. If not, I microwave it at our house and throw it into a baggie. By the time we reach our destination, the potato is cool enough to eat. Also, steaming in the baggie makes for easy skin removal. You can then cut it up or mash it up and season it with salt and pepper.
There are usually enough fruits and veggies that are dairy free (get to the relish tray early!), along with the cranberry sauce and jello salads (as long as they don't have cream cheese, of course). Check to make sure the turkey hasn't been basted with butter. Otherwise, it's usually fine. When I don't know much about the foods being served, I will pack some precooked chicken or turkey and canned green beans as well as the bread, potato, and dessert.
If this is an immediate family event, you can usually check with everyone ahead of time about what they're bringing and what the ingredients are. Some people are happy to alter recipes, and some are not. I choose not to push the issue and just bring lots of our own food. This is each parent's personal choice. If someone is interested in substituting dairy-free margarine for butter, I am happy to give them safe brand names. If great-grandma's stuffing has always been made with butter and American cheese and we are by god not changing that for anyone, fine.
As we all know, food can be a very emotional topic for people, especially at holiday time. Also, cooking dairy free (or gluten free, or nut free, or egg free, etc.) can be very intimidating. And, as many of us were taught that asking "What's in this?" is rude, it can be hard for people not to take offense. I really, really try to be understanding. I am so appreciative when people try to make things Katherine can eat, and I try so very hard to remember it is nothing personal when they don't. The flip side of this is that I hope they remember that we are talking about a six-year-old child. So I hope they aren't offended when she doesn't like the dairy-free dishes they so carefully made for her.