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I have a 6 and 7 year old and a 4 month old.
When my older two were 3 and 4 (and still today when they're home from school), we did lots of different kinds of sandwhiches that I would often make the night before (this made lunchtime easier and the sandwhiches actually tasted better). I would always use a hearty but soft whole grain bread and make combos like turkey, mustard/mayo and cheese; ham, mustard/mayo and cheese; roast beef and mustard; egg salad; tuna salad; chicken sald (made up from the previous night's rotissere chicken). If your kids eat lettuce (romaine counts as a veggie serving), top the sandwhiches with it the day you are serving (mine were too picky for lettuce).
Also, my kids loved crackers with different (protein-based) toppings like melted cheese, tuna salad, chicken salad, and pb & j (using natural pb...not skippy or jif).
And then there are tortillas.....we stuffed them with everything. I heat on a skillet for a minute, then stuff with cheese...or taco meat (I always make extra at dinner, portion it off and freeze to reheat) with cheese and lettuce tomato etc...or leftover rotissere shredded chicken(I portion/freeze as well) seasoned/heated up with some cumin, garlic salt/pepper....or scrambled eggs with ham/cheese. Of course you can stuff with any cold fillings you would put in a sandwhich, including something fun like fruits, veggies, hummous, taboleh, etc. Tortillas are great for the kids to get hands on as well.
One of my favorites though is leftover pancakes. I always make a surplus of multigrain (my own recipe) that we enjoy the next day too---and the leftovers I portion off with a bit of wax paper between each one and freeze for an anytime meal. The only trick there is to choose a recipe/or mix that tastes good cold or at room temp because I'm not crazy about the way they turn out reheated. We do the same with waffles. My kids eat them with their hands, like a cookie, for lunch at home or school or in the morning before school.
Nachos are another favorite (you can add bean dip, veggies, ground sirloin, chicken, etc. to the cheese).
Eggs....my kids love omelettes/fritattas (fritattas are easier to prepare since they finish in the oven and you don't really need to babysit).
Pizza bagels using day old bagels or from the freezer (when we can't finish them all from the weekend, I just slice and then freeze so they're ready for the toaster or oven).
Eggs...again...I also love to use my almost-expired eggs for hard-boiled eggs. They peel easier the older they are. I turn them into egg salad, throw in pasta salad, and my kids feel super special when I slice them in half and mix the yolk with mustard and mayo and turn them into deviled eggs. Also, they just love them as is. I always buy organic because the yolks have extra omega-3, which is great for their little brains (ps---Costco has a great price on them).
And my final bit of two cents is cold capellini (angel hair pasta) from the night before that I lightly dress with olive oil/butter. This happens to taste great the next day cold or reheated. You can use any shape pasta, and smaller bite-size shape can turn into a pasta salad with raw veggies, vinaigrette, hard-boiled eggs, etc.
To all of those dishes we add fresh fruit/or veggies (though my kids were never great about veggies).
I always found that prepping as much ahead of time and keeping in fridge or portions in freezer (if possible) made me feel much more motivated to offer a variety. Most of the time I do it when I am wrapping up leftovers from dinner, anyhow.