I agree with what others have said about the back needing to be attached to the top here and there in order for it not to billow out separately. Because fleece is very stable, you don't have to do a whole lot of quilting. It could be tied, but I find that more tedious than machine quilting. You could just stitch a bit every 6-10 inches in every direction, perhaps a bubble or loop and then travel to the next one in a wide arc.. If the baby is a girl, maybe a free-motion flower here and there. They're fun to do. Watch some youtube videos. It's important to make a swatch of the type of fabric you plan to use, and test/practice. Also, the flannel should be washed first because it will shrink much more than the fleece.
Top stitching around the edge of a birthed quilt is a good idea, but not sufficient to keep it neatly together. Just picture a toddler dragging it by the middle of the top layer, and you'll be able to see why.