What I do with my daycare kids is that they get a plate with a bit of everything on it and if they only eat a bit then fine, they are done, but if they eat it all and want more, they can have more. I give the older kids the same amount to start and the same with the younger ones. I state in my policies that I don't force anyone to eat, but they either eat what they're given or they go hungry until the next meal.
The kids learn fast that I will not give in to their groaning or comments that they don't like something.