The single most important nutrient growing kids need, is healthy fat. I make sure my son eats ample amounts of avocados, egg yolks, butter, coconut oil, fish oils (especially fermented cod liver oil), grass-fed animal fats, yogurt, raw milk and kefir. After fat, make sure protein is covered. It's a good idea to give your kids different sources of protein with every meal to make sure they get a full spectrum of amino acids.
With the guidelines of lots of fat and protein for all of my son's meals, the other half of his plate is an assortment of vegetables to provide necessary minerals, vitamins and fiber. Carbohydrates are your least valuable food- as their main nutrient content is starch- which is the least important of all "food groups" if you want to think of them in this way.
Here are a few pictures of my son's lunches last week:
Sometimes the easiest lunch is just leftover dinner! I'll often pack up stir-fry, meat and veggies or casserole into Ethan's lunch for the next day. I've sent him with leftover chicken soup, and shepherds pie too. Two of my other absolute go to lunch snacks are homemade liver pate on crackers and full fat, plain yogurt with grated apple or berries.