Kids Down Jacket

Your rain jacket pattern might work. Especial if you have seen it on your kid, you will know how to alter it to make it work. I would think that would be lose enough to let the down loft properly. Another option is the thru-hiker Whitney down jacket. If they have an extra small pattern their instructions and step by step guide is very helpful. I used it this fall to make the jacket for this current winter. It came out very nice and wasn't too difficult to follow. As far as how much down you need, might not be much more than an ounce of 800-900 fill to cover your kid from head to toe with goose. For comparison my jacket has 3-4 inches of total loft around the body and 1 inch total in the sleeves and hood. I am a 5-10 male 160 lbs. This jacket only has 5.9 ounces of down.