Our kids are 6 and 8 so we're not sure how this would work with younger, but we made a deal with our kids. We'll only put on their plate the amount that we know they can eat and they're charged real money based on the amount they aren't willing to finish.
Our kids don't have a lot, but usually have at least a few dollars. They'll say "how much is this" if they really aren't liking it and we'll come up with an amount like $.75. Sometimes they'll eat to $.25, but after doing this for a couple of weeks they were eating all of their food without asking how much to not eat it.
Because we want to travel the world, we intentionally give them small amounts of food that they don't like and have them eat that as well. We want them to be willing to try new things and to learn to eat what they're given as there'll be times that we don't have a choice.