A big family gathering packs unfamiliar relatives, new smells, a change of schedule and an expectation of sitting still into one long afternoon. It is a lot to ask of any child, not a failure if it does not go smoothly.
A quick message ahead of time — "he doesn't love surprise hugs, a wave works great" or "she'll probably want to leave the table early, that's normal for her" — prevents an awkward moment in front of everyone and spares your child having to manage other adults' reactions on top of everything else.
The same rule as any large gathering: one room, known in advance, where leaving the noise is always allowed and never a big deal. State it before the day starts, not partway through when things are already overwhelming.
An unfamiliar table of new smells and textures is not the moment to introduce anything new. One dish you know is safe means the meal is not also a battle.
What time people arrive, when the meal happens, roughly how long it goes on for — the same predictability that helps every day helps considerably more on the one day the whole house is different.
Leaving on your own family's terms, with a plan you decided in advance, is not rude. A short, well-managed visit usually goes better for everyone than a long one that ends badly.
A brief, plain explanation — "this genuinely helps him get through the day" — is enough. You do not need to justify it further, and you are not obligated to relitigate it each year.
Not if that is not realistic for them. A shorter time at the table, or eating a little separately and joining for part of it, is a reasonable version of participating.
Part of our guide to Big feelings & belonging.