The baked potato in a mitten is a good idea. You could also send along a thermos of chili (canned or homemade) to top the potato at lunchtime. A thermos can really get a workout during the cooler months.
Bake up some canned biscuits in the morning if you have time (they only take about 12 minutes), wrap those up, and pack a thermos with chunks of beef or chicken in gravy to pour over the biscuits. You can prepare most of those things from a can if you simply don't have time to cook! I guess you'd have to watch out for sodium, though, if you're using jarred or packaged gravy, but once in a while probably won't hurt.
My mom used to bake a turkey pot pie in the morning and put it in a container for me. It was a good idea, but it got cold and soggy before I could eat it. Maybe a Hot Pocket-style pot pie wrapped in foil would stay warmer and crispier until lunch.
Using the thermos again, put in some meatballs (the frozen kind, or even the frozen veggie version) with some sauce and pack a spoon to dip them into a hoagie roll. Put in a string cheese and the kids can make their own meatball sandwiches for lunch. You could even use chunks of Italian sausage. This would probably be good for slightly older kids, not the kindergarten set
Burritos could be heated up in the morning and packed in foil. You could make layered dip (refried beans, etc.) and pack some corn tortilla chips for dipping. The dip could be made the night before and just left to chill in the fridge.
spread refried beans in the bottom of a container
Mix taco sauce with sour cream (to taste) and spread on top
Add cheese, salsa, etc. I usually just add some drained Ro-Tell and green onions, but those could get stinky.
Pack it with a cold pack if you're worried about it staying cool until lunch. Pack it with tortilla chips and any veggies the kiddies will eat raw.
Pretty much anything wrapped in a tortilla - refried beans, PB&J, turkey...
For younger kids, I think anything cut into an interesting shape with a cookie cutter is going to be a hit. Cut the sandwiches or meat and cheese slices into animal shapes and the like, pack some crackers, and let them make their own Lunchable thingies.
It really depends on what your kids will eat, what they have time to eat, and how much time you have in the morning (or evening). It also depends on how much processed foods you are willing to let them have. There are lots of frozen products that will reheat in the morning and travel well wrapped in foil, but you might not want the kids to have all those preservatives. But once in a while wouldn't be so bad.
Send breakfast for lunch, even. French toast sticks, or the biscuits with a thermos of sausage gravy. The Vegan Lunch Box lady has a ton of great ideas that can be adapted for meat eaters, including the meatball sandwich and the biscuits and gravy lunches I mentioned. I just mentally swiped those ideas and added meat instead of the veggie version. Shh! Don't tell!