People in India were the first to grow cotton, about 2500 BC. It took four thousand years to spread cotton growing from India to China and Europe. But wherever people were, when they did get cotton they really liked it.
Then let the children come up with ideas for what to test. What would make one piece of clothing better than another? Is one of the pieces more waterproof? (They could try pouring water on them and measuring how much drips through in a certain amount of time) Is one of the pieces thinner or thicker? Does it matter whether it is knitted or woven, or how tightly it is woven? If they look at the cloth under a microscope, do they notice any differences? What might those differences mean for clothing? If they compare their own results with those of the other people in the class, do any general conclusions result from that? Can they graph their results? What kind of cloth would you want for hot weather? Cold weather? Rainy weather?
Cotton and Silk, by Jacqueline Dineen (1988). Easy reading.
Cotton, by Guinevere Healy-Johnson and Nancy Shaw (1999). Also for kids.
Cotton Now & Then, by Karen B. Willing (1996).