What a wonderful keepsake for your son and daughter-in-law. Love the colors.
I would definitely use the flannel you suggested. So warm and cozy. Whether you sash or not might depend ultimately what size you're going for. Without seeing all your blocks together, I'm thinking a tonal that reads solid, in a color that complements the room where it'll used most, would put the focus on the blocks. Then maybe a scrappy border repeating the fabrics from the quilt?