My kids have twin ikea down comforters that we use in winter. I just put them between their top sheet and normal bedding quilt without a duvet cover. I was them at the beginning of the season and once or so during the season and at the end of the season before storing them. Since they are twin size they fit in my washer and dryer (individually) just fine.
They work great and keep my kids warm. I've been very happy with the quality and together with our other warmth-inducing strategies such as space heaters before bed to warm up the room, flannel sheets, and fleece pajamas my kids have not complained about being cold at all even though at night we keep our heat at 60 and my DS's room in particular probably goes down to 50 because it's an old addition with 3 exterior walls that are very poorly insulated.
M, mommy to A 2005, E 2007, and L 2010