This was in a collection of short stories I read, probably in the 90's and I've been looking for it for a long time now!
As I recall it, it started with human soldiers on what appears to be a poor, technologically backwards planet. An alien child holds a doll that is strangely appealing. A soldier offers the child a chocolate bar in exchange for the doll, and the child reluctantly agrees. The doll is given to the soldiers daughter back on Earth and all her friends immediately want one.
The dolls become a huge craze, with millions being imported. Once enough of them are in place they activate and kill the humans by injecting them with something in their sleep, wiping out humanity.
Back on the alien planet, we see a secret base, and are shown that the aliens are not technologically backwards at all, and that this is not the first time they've used the dolls.