I really used to struggle with this also. But what can you do, force feed them?
And feeding the foods that they "want" but are nutritionally empty anyway, is only going to continue the cycle.
In my house, I make it and I serve it. Your choice from that point on. I will encourage, and praise someone trying new foods. For toddlers and up, I begin to enforce the "finish your plate and you can have more XXXX". For babies, I am less strict on that rule. I try to remember that kids often go through a phase in which they seemingly exist on nothing at all!! And then, usually a few weeks or so later, you cannot fill them up.
I think what happens is we all get sooooo stressed about this food thing, which the kids sense, and it just compounds.
And I would actually say the opposite about liquids... too much milk, juice, etc can fill a small tummy up and therefore they won't eat. I agree that you can never have too much water to drink, though.