J.M.
We just had a pediatrician come talk to our playgroup and she talked a lot about serving size and not so much in measuring everything out, but first you child's stomach is aobut the size of their fist. 2nd, it's okay if they don't clean their plate. Children will feel hungry, then eat just until that feeling subsides, NOT until their full. if we provide a balanced meal for them, they may not always eat everything, but be consistent at providing it and what they didn't like a week ago may all of a sudden be okay. She also stressed to make sure they aren't grazing on snacks all day. carrying around those "no spill" cups or bowls of cheerios are the worst thing you can do to sabotage their appetite for meal time. A Day might be: Breakfast, a snack mid morning, Lunch, Snack after naptime/mid afternoon, dinner, snack just b4 bed.
She said snacks should be no longer than about 10 minutes and be something healthy. cheerios/crackers are empty calories that in the long run are very good for kids. Don't let them carry it around. If they refuse to sit that the table or have the snack then they will need to wait until the next meal. Thus, we teach them, how to manage their hunger.
it's a lot how our society used to live, before vending machines, mcdonalds, convenience stores where we could graze whenever.