The holiday season can be extremely stressful for kids. Too many parties, visits with overexcited grown-ups, and foods that they don't usually eat.
Your job at this point is to protect your kids from things that are actually going to hurt them, and look away from the rest of it.
What that means is that you need to protect your kids' personal space. Even if your mom loves to kiss your daughter when she sees her twice a year, you can't let her if it scares your daughter. No one gets to tickle your child unless the child requests it. No passing the baby if the baby's not clearly into it. There are all sorts of things you can teach your kids to "connect" with adults that don't involve compromising their personal boundaries. My favorites are blowing kisses and giving high-fives (which are also cute and precocious-seeming). If your child is too young to do this, you can always pinch her to get her to yell, then claim all the overstimulation is making her cranky and she needs to go into a quiet room to chill out. (If you start teaching a baby the sign for "milk" from an early age, you can discreetly flash the sign and he will start fussing to eat and you can leave the room. Score!)
Let me repeat this again--You have to protect your child's physical boundaries. If you don't, you're teaching your child that it's OK for him or her to be violated in some way, because you're there watching but allowing it to happen. Blame it on anything you want to (your kid's coming down with a cold, your pediatrician, "this nutjob on the internet who writes an advice column"), but get your kid out of an uncomfortable situation no matter what.
For some kids, their physical boundaries also include the things they eat. I've been lucky in that both of mine have stomachs more like the stomachs of goats, so eating too much sugar or really rich foods doesn't have much effect on them. That means that's one thing I just let go of at events, and maybe hear the eating report later, or maybe not. But if you have a child with any food sensitivities, you have to be on guard. I do not envy you, because there are whole categories of people who think all food issues are made-up, and who will try to push you and your child.
I suppose one way to be proactive about dealing with jerks who try to push you on your child's food sensitivities would be to bring an epi-pen, and at the beginning of the event get up and ask all the adults to watch you teach them how to use the epi-pen "just in case someone accidentally gives Ethan something with eggs in it" or whatever the problem food is. Seeing the actual epi-pen might make them think twice about pressing the issue. I'd love to hear other suggestions from BTDT food issue parents.
The unrealistic expectations issue is a big problem. You end up having to spend a lot of time with people who have no idea or recollection of what's normal behavior for young kids. Tensions are high with relatives spending so much time together anyway, and then add in stresesd kids out of their normal environments, and it's a recipe for criticism, hurt feelings, and tantrums all around.
The best suggestion I have for mitigating this is simple: Go Outside. Go outside several times a day, and walk (or run) around with your kid. Being outside (even in super-cold air) is good for them, blows off some steam, gets them out from under the vigilant eye of the older generation, gets you out from under that scrutiny, and is impossible to argue with. (Who could say anything bad about taking a kid out to get some fresh air and run around?) If you spend enough time outside your child might sleep a little better (even in the too-small pack 'n' play or in bed with you), and you can settle down a little, too.
My other technique is to treat any egregious statement as if it's a joke. "I can't believe you let her eat potato chips before dinner!" and you answer "Oh, Mom, you're always so funny. Remember how we always used to eat so many potato chips at Grandma's that we couldn't eat any dinner? Good times." Or "You're still nursing that child? She's going to need therapy when she grows up!" and you answer "Ha! That's a funny one, Mr. Johnson. Could you pour me another glass of wine, please? I'm drinking the red."
If all else fails, just keep repeating to yourself one of the following mantras:
"I will not become my mother."
"Mizu no kokoro. Make my mind like water, and let it all flow through me without touching me."
"These people are not my real family."
"Only 185 more minutes until we can go home."
"Eff you, you effing effers."