HI Beth!
I think both of your ideas are great ones! I love to spend the first night of our vacation in a park enjoying the fireworks!
Your final choice may depend on your arrival time and resort. If you have enough time for dinner (or even snacks), then a nice stroll around World Showcase topped off by Illuminations is a great way to start. Or, if you running a little later, without time for dinner, then a trip to Magic Kingdom to view the beautiful Castle lit up at night with Wishes exploding overhead may be the way to go.
For a truly spectacular kick off to your vacation (if it fits in the budget of course!), you could also surprise them with a special viewing of either Illuminations or Wishes from aboard a boat.
Specialty cruises can give you a unique perspective on the evening's shows while in a relaxed, special atmosphere. The kids (and the young at heart) may also enjoy a little Character interaction to get things started. The
Pirate and Pals Fireworks Voyage is a great choice that is pretty affordable too!
Enjoy your trip! I hope you a wonderful trip!