You could get an insulated tiffin box or bowl, and it'll stay warm, but the cheese will solidify somewhat. If your kid is okay with the leftover piece of pizza before it's put away, or a quesadilla that has been sitting awhile, it might be okay.

That's a tough one, and I'm not sure it's really practical, unless kids eat lunch fairly early in the day.

You can certainly keep hot soups and stews warm in a thermos, but I don't think there's anything comparable for pizza or sandwiches except maybe a lunchbox size Wonderbag.

Good luck

There are so many better things they can have that might be easier for you to prepare too. I wouldn't want to be cooking pizza at 7 in the morning!

Pre-packed school lunches for kids often get into the food safety danger zone. 40F to 160F.

There is actually a device on the market which can be perfect for school lunches.

You can buy light weight, not too expensive personal electric refrigerator which uses what is known as Peltier thermal junction device for cooling. It has no compressor.

The most interesting thing about this refrigerator is that, by the simple flipping of a switch, it becomes a heater and warms the cold food and drink inside.

The only problem is you do need to plug it into an outlet.

dcarch

Here is a link that might be useful:

I have some of those fridges, they are all right for drinks, decent for travel, but are very unreliable for "real" food. They don't even keep cold food especially cold

We had one of those for road trips for a while...odd thing. Plugged into the cig lighter.
Worked well for the short time it did. (not hello kitty) Just a basic black thing. Felt a bit gadgety like sharper image stuff.

Not clear, but are you tying to keep hot sandwiches hot? Just not enough 'mass' or volume to keep it that way. Many insulator type things exist but need near boiling temp and liquid volume to keep it hot for even a few hours.

I like cold pizza. Not the thick gunky kind that goes soggy, but we often use flour tortillas as a thin pizza crust...just baked on a stone or cookie sheet with simple good ingredients, half what most normally pile on a pizza...get the crust nice and crisp...top bubbly...cool, then fold in half, then cut half like a quesadilla and fridge/or freeze. Fits in a sandwich size zip-lock.
Not a great example pic because of the pink...that is a thin sliced red potato around the edge...one of those pink all the way through babies. Gets crisp like a potato chip. (has to be mandolin thin)...probably red pepper, mushroom, prosciutto, gorgonzola...old pic. Never the same but we just use up what's in the veggie bin for a quick meal once a month on a friday and make extras for sat/sunday snacks. Often crack an egg or two on top.
They do re-heat well but also good cold...no different than cold quiche.

Just an idea if your kids like pizza and quesadillas. They could pick and choose ingredients. Have a pizza party on the weekend, then freeze the extra ones. Or bake not so crisp and do roll-ups.

My kids frequently take leftover pizza for lunch, and they like it cold. When we make pizza, we think ahead to lunch, and make the lunch pizza crust slightly thicker so that it holds up better in the lunchbox. We also let it cool a bit before putting it in the fridge--this helps keep it from getting too soggy.

For quesadillas, we also use leftovers from dinner. For lunches we make them with a top and a bottom and brown them up well, then let them cool before refrigerating for lunch. Like the pizza, letting them cool before boxing them up keeps them from getting soggy. We pack them cold with a little salsa if they want it. My kids like 'em cold, as long as they're also crispy.

I wouldn't try to keep them warm, they'll only get soggy and I'd be very worried about keeping food out of the 'danger zone'. It's far easier to keep things cold (and safe) with ice packs.

Cj