Okay, I just did some research, since I've been wondering about the pjs for a while. Here's the thing: the poly fleece pjs are not chemically treated with a flame retardant, instead the flame-retardant properties are actually added to the fibers of the polyester when it is made. That is why a retailer can claim that the poly material itself is inherently flame-retardant.
So, it is true that polyester fabrics will melt when exposed to flame. All synthetic pjs and loose-fitting cotton pjs sold in the US have flame retardants added. However, fleece poly pjs can have the flame retardants added to the fibers of the poly during the manufacturing process, and therefore are not "chemically treated."
I have no idea whether or not adding the fire retardant properties to the fiber of the poly material is safer than chemically treating the finished fabric or not. I wonder if they also do that so the fire resistance doesn't wash out over time?
The only pajamas that do not contain flame retardants of any kind are the tight fitting cotton ones that specify on the tag that they are not fire resistant.
Here is a link:
http://www.5minutesforgoinggreen.com...drens-pajamas/
It isn't a fabulously reliable source, but what she is saying makes a lot of sense to me.