Are you getting JUST the pinata? And did you choose "Use Item" instead of "Play with" a pet? Using it should yield 3 random Demi items and can include the pinata again.

I just used it and it said: You have gained: Demi Paper Clip Cream Demi Elixir Demi Pinata

"Playing" with it has a different result. If you used one of your pets that have a minion to play with it, nothing will happen because they are already happy with a minion.