My rule is 'I am not a restaurant', haha. I serve one protein, one bread item, and two veggie items at lunch for example. While I agree that children do not have control over much in their lives they must learn what they can decide and what is decided for them. That's called parenting or in our case caregiving.
So I ask them to do their very best and I serve a few teaspoons of each item, then offer seconds and thirds if needed, but I hate throwing food in the garbage. They can decide to eat one of the two veggies and I'm fine with that as long as they 'do their best'.
I do offer fruit after lunch, or a semi-nutritious item like corn nacho chips just as a few examples. I tell the children that they have to eat as much of their main meal as they can if they want their fruit. If they absolutely refuse to finish their lunch then I just save the fruit for afternoon snack. If they eat a couple bites of everything on their plate I'm happy with that and I give them the fruit. In this way I am giving them a choice - choose to eat as much of your main meal as possible or go hungry. A child will learn fast that a hungry tummy doesn't feel very good. And it's only a matter of 3 hours until afternoon snack time.
I don't believe in making children clean their plates and force themselves to eat too much because I think that's setting them up for problems later in life.