Welcome from Ontario, Canada. Polyester batt is harder to quilt on a smaller sewing machine and it also shifts. With two layers of flannel and the warm and natural batt, it should be toasty. Flannel and cotton batt tends to shrink when washed but if the backing is flannel also it will shrink together. I might use a 3/8 seam allowance rather then 1/4 because flannel usually has a looser weave and frays more.