Hello Sister Esther,
I don't know what the U.S. name for this fabric is, but I always make my nightdresses out of winceyette, which is a cozy type of pure, brushed cotton flannelette. It comes in charming, feminine, floral patterns, ideal for making up into modest garments.
If you search for images of "winceyette nighties" you will find many pretty examples like this one.
Good luck, and happy sewing, Sister!
YiC
AW