When my son was a baby (almost 22 years ago), he had a baby bag instead of a snow suit---the legs were split like a pair of pants so it would work in a car seat, then it all joined together in the torso area and went on up with the front ending just below his chin and the back long enough to serve as a hood. I think you could knit something similar that could be placed in the swing and your grandson could just slip in and out. I'm not a designer or a very good pattern maker, but I would think that if you had a sweater pattern for an adult, you could follow the instructions for the sleeves to make the "legs" of the blanket and then once those are done, attach them to a larger "body". Again, visualizing sweater pieces--it would be at the top of the shoulder, not the sides,and the neck would be sewn shut to form the between the legs area. If you knit it of wool on larger, say 10.5-13 needles, maybe it could be felted to size?