I know it doesn't answer the OP's original question, but I thought I would post a link to a short video of a flame test between standard polyester fabric and a polyester fabric that had a chemical added during the manufacturing process of the fibre to be inherently flame resistant: https://www.youtube.com/watch?v=OeNOy3w76Wk
Personally, I don't like polyester sleepwear as it is hot and doesn't breathe. Plus it burns and melts at the same time. Whether it has chemicals added during the making of the fibre, weaving, or after the fabric is made is less relevant to me, as it is a man-made fabric. While there may be some polyester fabrics that are inherently fire resistant, the fact that Carter's doesn't advertize loudly that their polyester fabrics used for sleepwear have NO chemicals added to the polymer matrix either molecularly (as the above fabric does in the video - it is permanently flame resistant), or through a process in the finishing of the fabric, tells me that they use polyester fabrics that have *something* added.
For *my* family, I choose cotton or other natural fabrics that have not been treated for flame resistance or retardance. I really didn't (and don't) want to get into a huge discussion about flame retardance vs. resistance, vs treatments of various fabrics here, as it really didn't discuss the OP's original question. Since her DC are beyond the age that Carter's, Inc. typically fits (I believe they go up to age/size 7), maybe some other brand suggestions would be helpful?
This isn't meant to be snotty or sharp; I am tired, my kids are bickering behind me and need to go to bed, and it feels like 120 degrees due to the wood stove. Since I work with fabrics (including those with flame retardants) professionally, I tend to look closely at what a manufacturer says or *doesn't* say about their fabrics, since there are often loopholes they can take advantage of.
OP - if you can sew, or find comfy cotton clothes labelled "not intended for sleepwear", those might be your best bets.
ETA: Here is another link to a fire retardant polyester fabric manufacturer that talks about the processes they use to make the fibers inherently fire resistant: http://www.libolon.com/flame-retardant-polyester-1.html
Edited by Blue Lotus - 12/30/12 at 11:26am