MontBell and others have made sleeping bags with no zippers, just a big drawstring for entry and hood. You would adapt that from an existing synthetic fill bag, removing the zipper and sewing up the side. Turn it inside out, sew across the foot to the length you want and cut off the rest. The drawstring hood opening on many bags would be big enough to get a toddler through. With a little luck you might find a really good bag with a bad zipper. I see them at the REI garage sale now and then, or someone here may have one.
The sweatshirt top idea is interesting, but not as warm. You could make an "elephant's foot" bottom to go with an insulated hooded jacket top by cutting the bottom off an existing bag in much the same way as I mentioned above. Wild Things makes a half bag for adults that hits about the armpits.