How warm?
When my kids were small I just used an adult sleeping bag and shortened the end with some bungie cord or I let them use my elephants foot. 4YO I would use synthetic 6 to 8YO I would use down I have a couple of surplus kids bags here that I am happy to let you have very cheaply. The tadpoles here are now too tall for them.
Very warm Postage will cost more than the bags if you were interested
The Kathmandu Pipsqueak when a bit older is a reasonable warm weather bag and the Orb for young teenagers, not much else around. If for serious walking above the snowline then I would recommend the Western Mountaineering Tamarack or the Semilite
http://www.westernmountaineering.com/sl ... s/tamarak/http://www.westernmountaineering.com/sl ... /semilite/I owned a Tamarack but sold it when I bought the Brooks Range elephants foot, a very good bag for short persons down to freezing when wearing warm clothing