My general understanding is that Fantasmic is more likely to be affected rain because it involves live performers, and if the ground is too slippery it's a hazard.
WoC, on the other hand, only seems to be cancelled for bigger storms. I don't have personal experience with this, but I've heard people say the show has gone on in light rain. I don't know how the dessert party would be handled in that situation though - if they would bring out umbrellas, or maybe pack up your desserts so they don't get wet until you're ready to eat them. The dessert party page does say this:
- Dessert party packages will not be refunded unless the show is canceled by Park management.
So it sounds like if the show is cancelled you'll get a refund.