Most of the flannel you will find at quilt stores is cotton flannel. It does shrink more than other cotton fabrics, so it should be washed before mixing with other fabrics. Children especially enjoy the texture of a quilt made from different fabric types. I mix corduroy and flannel in rag quilts all the time. As others have noted, starch can be your friend when working with stretchier fabrics like fleece or flannel.