Welcome, Lynda!
A Fantasmic! Dining Package is the best way to secure a great seat for this amazing nighttime spectacular! You've also made a great choice in selecting The Hollywood Brown Derby, it's where I've chosen a few times (and again in for my November vacation). I've also had the opportunity to experience last year's inaugural season of the Jingle Bell, Jingle BAM! Holiday Dessert Party - it's a lot of fun and our photo with Santa Goofy is one of my favorites.
No worries regarding the "Fantasmic! 1st Show" marked on your reservation, there is only one showing during your vacation dates and it is at 8:30pm. Occasionally there are two shows; when that happens all dining package guests attend the 1st show. You can select the highlighted link to view the showtime schedule, click on the
blue calendar box on the right side of the page under "Hours" and plug in your show date.
While your Jingle Bell, Jingle BAM! Holiday Dessert Party begins at 6:30pm, the actual show itself begins at 8:00pm and is approximately 14 minutes long. The problem is going to be getting from your reserved viewing location (which is nearer to the front) back to the Hollywood Hills Amphitheater before showtime for Fantasmic! Guests with dining packages are asked to arrive 20 to 30 minutes prior to showtime and you'll probably arrive just as it's starting. So even though you have reserved seating with the dining package, you might not end up with very good seats. While it's doable, it might be more immersive and enjoyable to choose the one that is more of a priority.
I hope this helps you plan, Lynda! Please let me know if you have any additional questions. With both of these experiences under my Ear Hat, I'll be more than happy to offer which one I'd choose :)
Faith