I use flanelette in winter and cotton in summer - you could use either, it doesn't really matter, but I find that in winter the flannie sheets don't feel so cold to the touch, so when you put them to bed, they don't wake up because their warm little head is being put on cold cotton sheets. I have 3 sets of cot sheets, but will get another summer set soon. You can always use a terry flat folded in half or even a towel for her head to be on, so if she does posset in bed you only have to take that and wash it and not strip the cot kwim?