The report in the journal Pediatrics says little sacks can be incubators for illness.
"There are different bacteria that can multiply in the lunch, such as E. coli, salmonella, staphoreas and some of them can produce toxins," said Dr. Pia Pannaraj, an infectious disease specialist.
The lunches of more than 700 preschoolers at nine child care centers in Texas were measured with temperature guns before they were served.
Thirty-nine percent had no ice packs inside, while about 45 percent had at least one ice pack. More than 90 percent of the lunches were found to be at unsafely high temperatures in which bacteria can grow.
To prevent bacteria from multiplying, food needs to be kept heated at over 140 degrees or kept continuously cool at under 40 degrees.
Small ice packs usually don't do the trick. It may take multiple ice packs.
If you don't have access to a fridge, Pannaraj says avoid packing tuna salad, egg salad, cold cuts and mayonnaise.
Pannarah says be extra careful about packing yogurt and cheese. Peanut butter and jelly is a safer bet.
Soups should be kept in cans. Foods get contaminated when they are transferred. Also, don't slice up fruit and make sure to wash your hands before you pack the lunch.