Lunch smells off by 1 pm in this weather. What do you pack that holds up?

Both children leave at 7.15 and eat around 12.45, so the food sits in a bag for five and a half hours. Since the rains started the curd rice comes back sour, the chutney with the idli tastes fermented by lunch, and my son has given up on sandwiches because the bread goes soggy against the filling. I tried insulated boxes, which seem to keep things just warm enough to make it worse. What do your children carry in July that is still fine at lunchtime? Practical answers, please. I'm not packing a separate ice pack for each child.