The heat can be fun for swimming but it is not fun if you leave food that should be kept cold out in the heat. Any time you leave food out in the heat you know you are asking for bacteria to grow... and grow it will! Many people brown bag it to have their lunch at work... many have no choice, many do it because they want control over what they are consuming. The trick is to keep that food safe until lunch time. Leaving a brown bag in your car is not the way to keep that lunch safely consumable!
My husband carries his lunch every day and doesn't have a refrigerator to keep his lunch in until lunch time so I use an insulated bag with reusable freezer packs. Not only does his beverage stay cold, I know his sandwich is safe for him at lunch time.
If your children are going to carry their lunch, I strongly suggest that you sew up an insulated lunch sack. If they carry their lunch only on the days that they don't like what is on the school menu one bag should be sufficient but if they carry their lunch every day, consider making a couple bags so you can toss one in the wash and still have one to use. With all of the novelty prints that are available, you're bound to find a fabric that will make a bag that is their "favorite" lunch bag... one they will always remember.