Of the one's I've made, and the one I'm currently making, I use mostly 100% cotton on both the front and the back. Sometimes I will include flannel in the top, it just depends on if it fits my color scheme since ALL of my baby quilts are scrap quilts they are usually made from what's in my stash.
Scrap quilts are my absolute most favorite quilts to make. And I find that with baby quilts I can use up small amounts of scraps that wouldn't fit into a much larger quilt because there wouldn't be enough fabric.