Fantasmic is a Hollywood Studios. Book a lunch (or dinner) at Brown Derby, and you can request a "preferred seating" pass from them. It's free, but you have to ask for it.
It's a great show, and the preferred seating means they will pick you up in a golf cart and drive you to the ampitheater, and then you get...well...preferred seating. It's a blast.
I would highly recommend it, but reading about your daughter's concerns, only you can truly judge if she's ready for it. Try checking out youtube for videos of it. The first year (when my kid was 5) she spent a large portion of the show in my lap hiding into my chest and peeking out. But, after the show she couldn't stop talking about it.
For the fireworks, make sure you plop down 30 minutes or so before the show, at minimum. My daughter and I like to grab a funnel cake and hot chocolate from Sleepy Hollow, and plop down dead smack in the center in front of the castle, about 20 feet back from the stage.
Another good place to watch the fireworks is a courtyard that is right before you get to Tomorrowland Terrace. If you look to the left, you'll see some stairs that lead down to a courtyard. People seem to forget that it's there...and many times you can grab a seat at a table and watch the show. Sometimes it's roped off, but I've done it many times. Only bad part is that it is near a designated smoking area, but it's very open air, so unless the wind is wrong, that doesn't bother me.
As a group of two, when you show up at Royal Table, ask for the window table. They will most likely have already assigned you to it, but there is a two top table in the center window that overlooks the carousel. It's a wonderful table to sit at in that restaurant, and me and my daughter do it twice every visit.
Be sure to get an autograph book for all the characters you are going to meet! I buy them at the park, but there are a lot of people who make them at home, and that seems like a fun project to me (it's just that, entering the park, buying an autograph book/pen/ears/pin, is now a tradition for me and my daughter...so...<shrug>
Have you thought about where you are going to use your Quick Service credits? All of the food is pretty good at Disney, but there are certainly QS places that stand out.
I would recommend the following places:
Hotel Food Court - Gotta have a Mickey Waffle!
MK - Columbia Harbour House, Cosmic Ray's (really fun if you sit down in the theater area), Sleepy Hollow (awesome prosciutto omelet, and Tomorrowland Terrace (if it's open when you visit). Turkey Legs can be bundled with chips and a drink for the cost of one QS Credit as well. I like to split a Turkey Leg with my kid while we watch the 3pm parade. It's far less crowded at Frontierland then over on Main Street, and they have Rocking Chairs you can sit in (if you can get one!)
EP - In futureworld there isn't really any outstanding quick service, outside of the Land pavilion, which offers a lot of variety. However, in World Showcase, most of the pavilions have a full service and a quick service option, so you can eat around the world! My personal favorites (for quickservice) are Katsura Grill (Japan), Sommerfest (Germany), Tangierine Cafe (Morocco) and the Yorkshire County Fish Shop (UK)
HS - The ABC Commissary is very good, as is the Backlot Express.
AK - Flame Tree BBQ and Yak & Yeti Cafe are both very good. Restaurantosaurus is ok, but it's pretty much the same menu/food as Cosmic Ray's at MK
If, at the end of your trip, you have unused snack credits, you can use them to buy candy and treats to give as gifts when you get back. I always pick up a few Mickey Mouse Rice Crispy treats as gifts for family and coworkers.
Also, be sure to use a snack credit to split a Dole Whip! You can find them at MK near the exit of the Tiki Birds.
5 is a great age for a first visit, and I'm sure the two of you will have a blast!
