so i guess that answers that. depending on where they live, choose batting that's appropriate for the warmth you think they need. dd may know whether they like to be warm-warm or cool.
if they like warm-warm, just prewash the flannel as above, and treat it as any other backing. if they like cool, batt it with the lightest batt around (there are some mighty thin ones, and they're not all cotton) or prewashed flannel. it's all in the desired warmth.