We have this thingy with a thermometer on it that says under 14 degrees you need pyjamas plus another layer of clothing on top plus 3 cotton blankets. Personally I think cotton blankets aren't warm enough so we have a wool blanket which I have doubled over. However, she's getting a bit more mobile now so I might have to go with a polar fleece sleeping bag with arms (saw some Bonds ones in Kmart the other day). Also we have an Onkaparinga wool proof mat (which is like a waterproof blanket underlay) under her sheets so something like might help if you don't want to go with a sheepskin underlay.