For many children, a rye bread meal easily becomes boring in the long run, so being able to give the children a warm meal in the packed lunch makes the meal experience much more exciting.
Warm the food thoroughly in the morning. It should preferably reach the boiling point so that all bacteria are killed.
Then you pour the food into your thermos lunchbox, which will keep the food warm until lunch.
Be aware that some thermos lunchboxes are really effective. This may mean that the food may still be too hot for lunch.
In that case, you can just let it cool down a bit before closing it.