The Wishing Penny
The Wishing Penny and Other Fantasy Stories.
Molly Maureen Mulrooney, sad that she hasn't been invited to Katy O'Sullivan's birthday party, aimlessly kicks a penny which cries "Ouch". The penny turns out to be a bewitched wee person that gives Molly one wish. But the wish must be positive and Molly can't think of how to ruin Katy's party with a positive wish.