Joann's has some of the flame retardant flannette stuff to make pj's out of but it feels & smells so icky there is no way I would buy it. Flannel nightgowns were good enough for me, mom, grandma, great-grandma, etc. and I felt lucky to get a new one.
As for the "flame-retardant" stuff we are supposed to use--aren't most fire related deaths caused by smoke inhalation? I would worry more about kids being allergic to the chemicals than being burnt by their own pj's.