Park Hopper Answer:
If you've never been to Disney World before and want to experience each park to its fullest on a short trip, you should give a full day to each one you go to.
Park hopping is really for when you're on a longer trip and want to bounce between parks, like when maybe you've got a few attractions you'd like to experience at Animal Kingdom in the morning, but you've got a dinner reservation at Hollywood Studios. That kind of thing. Your first time out, you should enjoy each park individually — don't bother with the extra expense of a park hopper.
Frozen Answer:
I've been vacationing at Disney World for 18 years, and I've never heard of any kind of substantial special event being held in the parks to celebrate the premiere date of any film. The only thing I can think of is when Pirates of the Caribbean: Curse of the Black Pearl premiered in 2003, but that premiere was literally held at
Disneyland, in the actual park. They gave out free buttons to attendees. If I were you, I would not expect Disney to do anything out-of-the-ordinary in the parks to celebrate the premiere of Frozen 2.
With that out of the way, I would highly recommend booking Mickey's Very Merry Christmas Party (
MVMCP). The seasonal parties are terrific and should not be missed — it will be a wonderful addition to your first Disney World trip. At the party, there is an exclusive Frozen stage and fireworks show called A Frozen Holiday Wish, which your 7-year-old Frozen fan is sure to love. I think the Frozen characters also have a float in the party's special parade... and character-meet-and greets may be available, but I'm not sure about that.
If you want to have a really special Frozen day on the 22nd, I recommend using your park ticket that day on Epcot and then leaving to go to Magic Kingdom (MK) when it's time for MVMCP. Your party tickets are separate from your park tickets, so this kind of 'park hopping' is totally and easily doable. So — why Epcot first, though? The Norway pavilion in Epcot's World Showcase is the central location for all things Frozen at Disney World. Your 7-year-old can meet Anna and Elsa there, and the shops are full of amazing Frozen merchandise. If possible, try to book a FastPass+ for the ride Frozen Ever After. It's very popular and FastPasses are snapped up quickly. If you're staying at a Disney resort, you can book 60 days in advance (September 23), and if you're not staying at a Disney resort, you can book 30 days in advance (October 23). If you have time and budget and are interested in character dining with Disney princesses in Norway, you should also consider booking breakfast or lunch that day at Norway's Akershus restaurant. However, keep in mind that Anna and Elsa don't meet at Akershus — it's just a nice restaurant right near all the other Frozen stuff, with a similar theme. If Akershus seems too expensive or like too much of an ordeal (character dining isn't for everyone) definitely check out the Kringla Bakeri Og Kafe in Norway. It's got some of the best quick-service food in all of Epcot. Then, you can explore the rest of Epcot, and when it's time, head over to MK for MVMCP for the cherry on top of pretty much the most amazingly Frozen day you can have at Epcot. I'm low-key jealous just writing about it.