I'm finding that children's school bags are becoming larger & heavier and one of the reasons seems to be the increased size of lunch bags/boxes. Here's a few options that will help create a little more space in your child's already bulging school bag.
Vintage Pillowcase Lunch Box tutorial from Oh, Fransson!
Belly Button Boutique added insul bright to her bag.
A cute embroidered lunch bag by The Purl Bee, similar the the Be Be Bold design but it doesn't have a lining.
Ayumi from Pink Penguin has put together a fantastic tutorial for a more detailed bag.
Use an oilcloth fabric so you can just wipe it down if you don't want to continually wash it.
Add a waterproofing spray to the fabric inside & out so protect against spillage stains.
Add an insulating fabric to help keep food cooler.