Anyway, your daughter would do just fine with a light mummy bag like Slumberjack's Super Guide, which uses a synthetic fill called Thermolite (made by DuPont, so not a no-name product). Cost is only $70 (www.slumberjack.com). The Super Guide also weighs a very respectable two pounds, eight ounces. And, it has a nicely tapered shape that's efficient without being too constrictive. In my experience, Slumberjack bags are slightly overrated temperature-wise, but in this case I think it would work fine.
At the top of the heap would be the Marmot Arroyo ($259; www.marmot.com). This is a superb bag rated conservatively to 30 degrees, complete with down fill that's longer lasting and softer than synthetic. It's also lighter; the Arroyo weighs about two pounds.
For any bag, you can easily reduce its volume with a compression stuff stack. This is simply a nylon stuff bag that has extra straps that can be cinched to squeeze the thing down. Kelty makes an 8-by-17-inch sack, which could handle any of the bags I just mentioned, that sells for only $17 (www.kelty.com). It'll take a bag that's the size of a small watermelon when stuffed and turn it into a cantaloupe.