Just make SURE it is insulated.
And, buy some "blue ice" ice packs to keep the contents cold.
And, make sure that the lunch bag is lead-free. Some of them have designs/print on them in which the ink has lead in it.
Just get a lunch bag... that your Preschooler CAN open and CLOSE, on their own. That is one of the most important things.
I work at my kids' school, and SO often, the kids CANNOT even open or close their own lunch bags. With their tiny hands.
So think of that too.
A lunch bag, HAS TO HAVE, a blue-ice ice pack in it.
I see so many kids at my kids' school, that have NO ice packs in it. Then their food at lunch is all warm and not good anymore. Parents put in perishable food in their lunch bags, and with no ice-packs. Totally thoughtless. And there have been times, that we had to tell the kids that their food is spoiled now. Because their parent put no ice-pack in their lunch bag. One kid even had dairy and left over fish, in his lunch bag. And NO ice-pack. Ugh. We had to throw it away.
EVEN if a lunch bag is insulated... you NEED to also put a blue-ice ice pack in the lunch bag. To keep the contents cold. Then it will last until lunch time.
And, make SURE you label your kids' lunch bag... with a Sharpie Pen.
Because, other kids may have the same lunch bag.
And also, young kids or any kid for that matter, often "forget" their lunch bag or even where they put it. Everyday, kids lose their lunch bags.
I see it myself.