Batting should be *extremely* light (split the batting, if necessary, or use a layer or two of double-washed flannel); otherwise the doll quilt will be stiffer than you may want. If you were doing regular quilting, I would also recommend using a finer thread and not over-quilting. Stippling with regular thread, for example, will stiffen the quilt way too much.
I hadn't heard of the foil idea. I would whip up a sample quilt in the desired size with foil in it and stitch through the foil to see how the quilt turns out. I think you would need to stitch through the foil as a layer in order for that idea to work. Seems to me, though, there could be drawbacks such as noisiness and eventual wearing out of the foil into fragments. Also I would think that machine washing would be hard on the foil (lots of flexing), so it would be more for show quilts than for play quilts.