Could you sort of sew something right into it- like a few of those glass marbles? For instance: take 2 squares the same size and lay one top of the other, sew seams straight down them making 4 strips, and put one marble inside each strip. Then when you sew the square into the quilt the marbles will be caught inside. But he could move them around from the outside. I think they would wash ok.
Does that make sense?