I have been wondering this too, my little girl is 18 months and even the sleeping bags up to 2 years don't have much room in them for her anymore as she is so tall so my only option is a couple of £30 odd pound grobags that go up to 36 months or try her with a duvet. She only uses sleepbags at night. At nap time she just has a cellular or crochet blanket that are breathable as I worry about blankets going over her face due to her being a wiggle bum but she always kicks them off completely and I don't want her getting cold in winter which I think will happen if I try her with a duvet as I doubt she will keep it on!
So hard! x