LL Bean has fleece sleeping bags. You can get those monogrammed. They're $29.95 right now and you could get it monogrammed with the color(s) she likes. I got one for my nephew and he used it for summer camping all summer, and now uses it at home just because it's soft!
http://www.llbean.com/llb/shop/43014?feat=sr
LL Bean has had some cute "regular" sleeping bags in the past. My friend's DD has one that's a "jungle" type print. I haven't seen one like it in a while, though. Mine is purple inside and out. If you want to get her a real sleeping bag that's also cute, that's what I'd do. It will last her a lifetime. The LL Bean Camp Bags are more expensive than you're looking for, but they come in great colors, and you can get them with a flannel, poly/cotton (like a sheet) or a fleece lining. They are NOT slippery on the outside like the regular Coleman ones are. I love mine. I've been sleeping in Coleman and "Hilary" (Sears brand) sleeping bags for 20 years, and the LL Bean one is SO much more comfy, easy to pack up, and cute. I just have a 40 degree bag and it's great for camping or for a sleepover.