I know what you're talking about! This was out first year without a baby or toddler (our youngest is 3 1/2) and it was by far the most enjoyable Thanksgiving in the last 5 years.
Definitely get a couple of those portable booster chair/high chairs that strap onto a regular chair. Even if there are no extra chairs available, you can put them on the floor. These are sooo worth it. We even kept ours because so many friends have tots that use them when they come over.
Also, taking turns is basically how we handled these big group meals for a while. We've spent many Thanksgivings at our aunt's house with one of us outside watching kids while the other ate. There are a bunch of cousins, so it always ended up that the dads would all take backyard duty at the same time while the moms ate, then the moms end up on duty when the football game is on.
Another tip about a kid not wanting to sit still to eat -- don't even attempt to make him sit OR eat. Eventually he'll come in and ask for food when he's hungry enough. And it's best if you throw out all pre-conceived ideas of nutrition, etc. There have been years where it seems like mine ate nothing but buttered rolls and pie all day. Just forget about it. It's ONE day. This was hard for us because we're self-proclaimed food nazis, but it was just less stressful to give up the fight for the day. Also, we started bringing a dish that we know the kids would eat. For example, they won't touch a green bean casserole, but they'll eat their weight in plain green beans. So, we bring plain green beans. This way we know that at least one vegetable will cross their lips before the day is over.
This doesn't mean that there's a total free-for-all where eating and behavior are concerned, we're just a lot more relaxed about everything.
Good luck, and I hope Christmas is less stressful!