This is a short story I read a few years ago. I do not remember exactly The impression I get is that this from 1950s/60s.
It involves an important person who is at a high risk of assassination. The person gets a toy as a gift. It seems to be a mock factory or something similar. It is checked by security and found harmless. He leaves it in his home, running. But as time passes, the time passes and manufactures tiny soldiers who end up killing the target.