I'm not sure how this went from "is it flannel" to flame retardant.
Cotton flannel is napped on both sizes. Slightly fuzzy feeling and looking. If in doubt, take a swatch of it to the fabric store and compare it to their labeled flannels.
A burn test can be used to help identify the fabric content if it is in question. Cotton will burn and leave ash. Synthetics will melt and leave a hardened edge from the heat. Wool will burn but there is a smell, very strong, like burning hair, which of course it is. Fabrics have different rates of burning also.
Flame retardant fabrics have been treated so that they don't burst into flame. They melt.