The toys, as far as I've seen, are brought home by their parents bought from somewhere that I've yet been able to ascertain. Shortly after the child begins playing with the toy, the toy either grows or is large enough already, it attacks and kills the parents, leaving the child alone.
I believe they are, perhaps, a version of the Patchwork Homoculi that the Hags have been sending out, where these are to strike at a domestic level as opposed to those that we encounter which have been sent out to either distract us or keep us busy and away from something else.
Take from that what you may, this is simply what I've observed and been able to peice together.