Something easy you can buy in bulk for cheap maybe? You can even make something simple as well, or even make a little plate of cookies or something.
I got these as little birdseed hanger gifts for a luncheon and they were pretty cute. They average about $4 a piece with shipping and she can make a listing for you for just a small amount:
But honestly, if I were a volunteer parent, I wouldn't expect to receive anything. The little card you give, or maybe even a drawing from the class would be plenty, especially since you already gave them a Christmas present!