While I do try to offer my kids healthy meals and snacks (with enough time between food to feel the sensation of hunger), some days they don't choose to eat everything. And some days, I just don't serve up the perfect meal. But don't worry, there's always tomorrow!
A single meal is not meaningful. It is the days and weeks of meals that matter. If your kids only pick at the veggies one night, try to include them in snacks (veggie sticks and dips) or their lunch box. Some nights my kids are not too keen on meat, if that happens I offer them an egg in the morning. If you think dairy was a bit lacking last week, then add some smoothies for lunch. My kids love grains and fruit, so I don't have to worry about those (protein and veggies are the ones we are currently working on).
Remember, kids eat more some days and less on other days. That’s ok, it’s their responsibility. Let kids eat according to their internal signals and grow into the body that’s right for them.