I use a layer of any flannel that, after washing, is too cheesy to use as the front or backing of a quilt. More and more often that would be Joann's or Walmart flannel.

I usually use poly in a baby quilt. It stands up to frequent washing, which maintains it's loft, and it is more lightweight than cotton. It's totally personal preference but the faster the quilt dries, the less it gets beaten up in the dryer.

For my flannel quilts I just used two pieces of flannel and no batting. Plenty warm but not real heavy. I would say for a baby quilt I wouldn't use it. I love to do rag quilts because they are QAYG and so fast and easy to put together.