Penny,
We just took our girls to the
Cirque du Soleil last week, and it was amazing!
The show really did take exactly 1 1/2 hour. So assuming you get out at 7:30 pm, and it takes you 15 minutes to navigate the Downtown Disney Westside crowds to get to the bus station, you could expect to get a bus by no later than 8:15 pm.
You really want to take the bus to the Contemporary resort, and then walk over to the Magic Kingdom from there. That is the fastest, and most direct route. The bus ride would take around 25 minutes, and the walk at least another 10.
So it is going to be a tight window to reach the Fireworks Dessert Party by 9 pm. And part of the fun of the party is getting there, enjoying some of the sweet treats, and taking in the great view of the park and central hub. So other transportation might very well turn the night from hurried to magical.
BOTTOM LINE: While you probably could make it there right at 9, you really should break out your wallet and spend the money to take a cab from Downtown Disney to the Magic Kingdom. You will shave a good hour off of your total travel, will not feel rushed, and will get the full value of your Dessert Party cost.
Have a magical (and delicious) vacation!