H.W.
Sandwiches are great. Your little ones can 'help' you make them (yeay, an activity ) and you can make a bit extra, in case they get hungry later on.
You can cut up carrot sticks and celery sticks to dip in dressing or a little salt, and have them in cups of water in the fridge, ready for you to put on a plate. My son loves red pepper slices with some hummus.
Hard-boiled eggs are good if your kids like them. You can eat as they like (some kids are very particular about yes/no yolks) and make as egg salad for yourself.
You can slice up a bunch of cheese and have it ready-to-go in a container. Kids like cheese slices on bread, crackers, sandwiches. Whole grain cracker 'sandwiches': lunchmeat cut smaller, cheese slices, peanut butter, jam, pickles, cream cheese, olives.... whatever floats your boats that day! Kids love these 'make it yourself' meals.
Beans and rice burritos: make up a bunch to put in the fridge for a couple days. Let your child choose what they want on their own (olives, shredded cheese, mild salsa, avocado slices....whatever suits) You can pack each of their burritos in separate stacking containers so that you can pull out their whole meal at once. Eat one serving the day you make them, and a second serving a day or so later.
Cut-up apple slices to dip in yogurt. (Offer the yogurt as a dip, not an entree, in a small dish.) Any cut-up fruit they like to eat... have it ready to go. Stonefruit is also delicious (plums, peaches, nectarines) and don't need a lot of prep. Just let them ripen in a paper bag.
Quesadillas are fun. Make in the morning and slice up to store for later.
As previously suggested, any leftovers are good. You can do a quick couscous with leftover veggies/meat chopped up fine and add some crumbled feta.
We sometimes make a soup: broth cubes, simple veggies my son likes in soup (fine dice so it's easier to eat them than pull them out) and a handful of star pasta, maybe some tofu for me.
By and large, though, lunches at our house are usually sandwich or soup and sandwich affairs.But you have a lot of possibilities with sandwiches too!