If you like the idea of denim, there are many options. Denim comes in lots of colors or you could use any of the bottomweights - rodeo, trigger, sportswear, etc. If you want something that has a bit of water resistant feel, look thru the babyville fabrics, They have a bit of a 'vinyly' side. Any, you could simply use a cotton and give it a finish of scotchguard to help it not soak in the grass dampness. If it is a special quilt to be used for picnics, you could also give them a same size bit of the thinnest gauge vinyl with the quilt to lay on the ground first. Roll it and tie it with a matching ribbon.