In general, I don't think it's worth worrying about. Most of the time, I figure if a kid doesn't want to eat his food, he is not hungry. So he can eat what he wants, which might be a little or nothing, and then leave the table.
If he comes back later hungry, maybe there is something about the dinner time he's trying to avoid - a problem with dad or older brother, say, or he's too wound up from school.
Would I make him something at that point?
Probably not. Pretty much at my house, the rule is, the chef makes dinner once. If you want to eat later, you need to do it yourself, with some support from the chef if you are an infant of course, but if you want to eat another time, you need to learn to do that yourself, and clean up after, and that better be pretty thorough, as the chef is pretty picky about her kitchen. And there is a list on the fridge of what's available for starving children, and it better come off that list.
As you can imagine, that's all a lot less appealing than just sitting down and stuffing your face and letting someone else do more of the work. So that doesn't happen often at my table.
What if they aren't eating a balanced diet? Well, all the experts say, it will all even out over time. If they don't drink milk, they might like ice cream, say, or frozen yogurt. If they don't like vegetables, they probably will eat a banana, or maybe a carrot.
At a certain age, many kids, their growth rate slows way down, and because of that, they tend to eat quite a bit less, some kids will even get rather thin. That pretty normal, and they will change their growth rate again later. Many times, parent's expectations don't change gear when the kid's growth rate changes and slows down.
When a kid doesn't eat, first of all I ask him if he has a stomach ache or otherwise doesn't feel good. If he feels sick, might need to not get loaded down or - well - we all know what will happen next.
If the problem is he doesn't like the food served, I know that quite a few younger kids, are actually, really physically nauseated by trying to eat different textures they're not used to. So they often don't like a lot of things. If they get a little urpy once, their stomach tends to remember even if they don't.
Generally, though, I find that kids try a great many things very easily, when their parents are adventurous eaters and try a lot of different things and are just laid back and not nervous about what they eat. We make a huge deal of being adventurous at my house and kids tend to really get into the spirit of things, especially cooking, choosing menus, etc. If they don't like something, I usually laugh and say, 'You're a BABY EATER', and they get mad and pop it in their mouths and yell, 'I AM NOT!'
I'm kind of a chef, so if a kid doesn't like something, I am a lot more interested in getting them to broaden their tastes than I am in getting them to 'clean up that plate because I told you to'...unlike the poor beleagured parent. Usually, if they are willing to take even a tiny taste, they get a big round of applause here, and everyone says, 'what's it taste like? come on, what do you think?' Usually some comment will be made about how MOST kids don't try that til they are MUCH older, LOL.
I had a kid dropped in my lap once that had 'an eating problem' at 2 1/2. His mother handed me a gigantic dinner plate piled high with about two pounds of shredded white chicken meat with nothing on it, and said 'He HAS TO FINISH THIS AND HE CAN'T HAVE ANYTHING ELSE TO EAT'. The kid looked up at me with an expression that said, 'Could you please just kill me now instead?' I looked at the food and almost ralphed myself. It was NOT attractive. It looked like something Richard Dreyfuss made in 'Close Encounters'.
Sometimes it's easier when the worried mom isn't hovering over one, and sometimes, well, you just need to be a little creative. Little ones seem to eat an awful lot when they're sitting on your lap and you're both eating off one plate, and watching a movie. Of course many hassled parents don't have the time, but I sure do. So if you got a bad eater, bring him on over.