“Birthday Boy,” by Amy Lukavics
Appeared in Unnerving Magazine, issue #6 (it’s the first story in the issue)
Maybe 3,000 words? I’m hopeless at estimating these things
Clever, and the mother’s emotional arc feels believable, at least to me. Of course, the story cheats by withholding information that the point-of-view character knows, but I think it gets away with it.