Longstrider, up above (http://community.livejournal.com/alt_fen/3354.html?thread=31514#t31514), raises a wonderful theory that Muggleborn children are taken and replaced by changelings, perhaps in good old faerie fashion. Combine that with the Squib-swap theory and we might have something really interesting . . . Maybe we have a complicated swap program, part voluntary and part involuntary.