The struggle is real with kids and food, but especially at holidays when most of the food may well be different than what they are used to seeing.
You don't want tears at grandma's house. You don't want children announcing that they hate what is being served. To save the day, teach kids to compliment and thank the cook: Thank you Grandma, it looks good. Amazing how that one statement makes the picky eater look like a super star gourmet. Even if they don't eat much, they already said it was good. Everyone is happy. The child looks like a genius.
Teach them never to criticize the food. If they hate it, they need to keep that thought to themselves.
How you cope with the picky eater depends mainly on what works for you, but here are some ideas.
Consider presenting children with a challenge food. That's one bite of something they hate or never tasted before. Just one small bite, like three peas. If they eat the challenge food then they are finished with dinner. It is especially helpful if kids are competitive with each other. Then the challenge food is a contest. You can increase the amount of the challenge food once they have tried it 10 times or so.
Don't make a separate dinner for picky eaters. Put the family dinner on their plate, with small servings. Experts advise not forcing a child to eat everything on the plate, but you can enforce some level of cooperation. They must eat a few bites of what you put on their plate, even if it means you sit with them and coach them through the bites. Mouth, chew, swallow, take a drink.
If your picky eater won't eat enough to fill them up, then you might make their favorite (microwaved chicken nuggets!) after dinner. But the key is they have to eat some of the family dinner.