Here's the basic idea: pick one protein, one starch, and two fruits or veggies. When I've been strategic about pre-made foods (aka leftovers), it works instead for kids to have an entree that covers multiple categories, like soup or pasta with sauce, and to round that out with a side dish picked from the list.
In a pinterest-perfect world, the kids would spend 30 minutes helping you prep, bake, cut up veggies, and portion bulk items on Sunday afternoon, so that they can easily grab a tub of carrot sticks and a muffin all week long. An alternative excellent plan would be to make lunchbox selections the night before so kids can help with any prep or portioning that can be done in advance at that time. Even with the best planning, certain things will still need to be done in the morning, though. For example, if they prefer their apples sliced, and you're a sucker for getting them to eat cheap and abundant produce.
A note on my organization: Things that require no prep beyond putting in a container are at the top of the list. Things that require more extensive prep are at the bottom. Many of the items that require more prep can be made ahead and frozen in individual portions. I've got a few fresh fruits and veggies that I almost always have on hand listed individually, but I also have spaces on the list to write in whatever fresh or prepared fruits and veg are handy this week. Also, it is clearly just fine to combine items from the list to make an "entree," should one desire. For example, peanut butter can be combined with bread to make a sandwich. Or, you know, this thing, if you prefer.
Lunchbox Mix and Match
Cheese slices or sticks
Peanut butter, nut butter, or sunflower butter
Nuts: Cashews, walnuts, almonds, hazelnuts
Ricotta- or yogurt-based dip
Beans (baked, refried, plain, or seasoned)
Hard boiled egg
Pre-prepared meat (pulled pork, steak strips, diced chicken, etc)
Chips or tortilla chips
Muffin or quick bread
Cooked grain: couscous, quinoa, wild rice, etc.
Fruits and Veggies