For my grandkids I got The North Face Tigger. It's synthetic, very washable, relatively light and warm down to (or a little below) freezing. Your 5-year-old won't outgrow it for another 5-7 years, at which time she'll be ready for an adult size bag.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey