Steve Hockensmith is one of the best mystery short story writers active today and one of the best even if you go as far back as Poe (which is roughly about as far as you can go in this field). This story is one of his Christmas stories, and those are always a treat.
In this tale, Karen and Ronnie (ages 9 and 6 respectively) are debating the existence of Santa Claus. Karen goes on a hunt through the house to find evidence to prove there is no Santa. If she can only find the gifts she knows her mother has hidden somewhere...But what if her mother, under the influence of "Cousin Rick" the man who showed up and took up residence in her mom's bedroom, has forgotten to buy any presents?
After all, Cousin Rick is just the type to think buying presents for the kids is a waste while the money could be spent on himself. An evil, self centered man with no use for the children.
No worries, however, more than any presents, Karen finds something that might just get Cousin Rick out of the picture for good. But what? How? Read the story. It is in the January 2008 copy of Ellery Queen, and I will send my copy to whoever asks for it first.