Do a dance emote at Innkeeper Saelienne, and in exchange you'll receive Darnassus Marzipan!
Even though you're trick-or-treating on behalf of a child, you still need to perform a trick for the treat! We can't very well change the tradition, now can we...
I believe the cost of a piece of marzipan nowadays is to bust out and DANCE! That's right, let me see your moves! Dance for me, and I'll give you some tasty marzipan!