I've used cotton on one side, and nylon on the other side. And a simple strip of velcro holds the wrap closed.
When the sandwich is wraped up, it makes a nice package. The wrap also serves as a placemat when it's opened. Then when their lunch is done, they simply shake the crumbs out, and fold up the wrap and stick it back in their lunch bag. Their mothers can throw them in the laundry when they get home.
I finished up 10 of them today. I'll call them prototypes for now... The 5 kids can each test them for the rest of the school year, and give me feedback. If they like them I'll make more for them in the fall.