I'd make a test block out of the unwashed fabric, wash it, and see what happens. Flannel shrinkage is a problem if the block is not moderately quilted. When flannel is moderately quilted to the batting and other fabrics, the batting becomes the controlling factor in shrinkage. In other words, the flannel cannot shrink more than the batting in this situation. However, if you are just sewing an X through each block, that may not be enough quilting to keep the flannel from shrinking in the unquilted areas.
Creating a test block will also help determine if there will be bleeding from the pink and fuchsia fabrics and whether the white flannel picks up that bleeding. If the test block is okay, I would go ahead without prewashing.