The nightgowns are lovely!
You will never find nightgowns like this made out of cotton in stores unless the cotton has been treated with flame retardant. This is because air can reach both sides of the cloth. If the cotton comes into contact with, say, a space heater long enough for combustion to develop, the child will be enveloped in flame before anyone can move to put the fire out. I saw a demo of this on tv. They touched a match to the bottom of a cotton nightgown on a mannequin and the mannequin was engulfed in flames within a couple of seconds. They did the same to a polyester nightgown, and the fabric turned into gummy plastic -- also not good because the plastic is sticky, but at least the garment did not erupt into flames.
Untreated cotton can be sold in children's sleepwear only if the sleepwear is designed to fit close to the skin. In this case, oxygen can reach the fabric only from one side, so brushing against a flame will not result in the child being engulfed in a flash fire.
This is why the bolts ends of flannel in places like JoAnn's are always marked "not suitable for children's sleepwear".