Hellobee Boards

I would worry a little about bacteria growing if food is kept at a warm, but not super hot, temperature for that long. (But I'm a little extra cautious about that stuff, so YMMV) Could you set out the thermos and portion out the food in the fridge the night before in a microwave-safe container so that it's just a quick microwave and dump?
We use the thermos brand that are short with a wide mouth for sending food like that for DS. https://thermos.com/collections/food-jars/products/stainless-king-food-jar-16oz
If you put really hot water in to heat up the inside while you heat the food, and then empty and dry it out, the food stays warmer longer. It says that it will keep food hot for 9 hours, so that's pretty close.

I'll third the "microwave and dump" option. We do exactly this. My 10 and 6 year old really only use their thermoses for pasta meals or french toast sticks....both of which I personally think end up a little soggy and not super hot after being packed around 6-7 AM and eaten by 11:30 AM. We also use the "boiling water and wipe" method mentioned above.

My 8 year old eats much more adventurously (soups, curries, etc.), and these items fare better in the thermos than more "kiddie' items, IMO. So may depend what you're packing. But no, I would not leave out overnight in our THERMOS brand containers unless it's something like oatmeal maybe.

If Yeti or Hydraflask or Otterbox makes a food container (I've never looked), they might have an option that works. Perhaps something you could move onto if it turns out your little really enjoys the hot foods and you want a higher-end option for longevity.

Just saw this. We bought a Rigwa Life food bowl thermos. I googled and googled until I found a steel outer thermos bowl that had a plastic (BPA free) microwavable inner bowl. The inner plastic bowl has a silicone lid that's pretty leak proof BUT we don't put full liquids like soup in there. But I've used it successfully with thick liquidy things like chili.

So what I do is pack the foods the night before and put it in the fridge. Then DH microwaves both bowls in the morning for 1-2 minutes and puts them in the thermos. The bowls looks HUGE but they fit in a standard zippered lunch box.

The food is still warm at lunch for the girls. They are pricey but we've been using them for 1.5 years and they are still going strong.