First trips are so exciting!
If you only have two days to spend in the parks, then I would focus primarily on the Magic Kingdom. I think you will find more activities your 3-year-old will love concentrated in that park than in any other. Depending on the height of your child, you may even be able to enjoy a first roller coaster ride on Goofy's Barnstormer! If he/she is at least 35 inches then go for it, and be sure to mention to the Cast Member running the ride that it's your child's first ride. My girls got a special treat to commemorate the event.
For the parade & fireworks I don't think there are any particular viewing places that are necessary. The biggest thing is to position yourself in front of the castle for Wishes (fireworks), as the show really is designed around the castle and just isn't the same when viewed from other locations. We generally stop in the street on Main Street, watch the show, and then turn around to exit when it is over.
If you want to visit an additional park, I'd go to Animal Kingdom. My whole family loves this park, and my daughters especially love The Boneyard playground. Even my husband & I have been known to enjoy the slides there!
Kilimanjaro Safaris is another favorite, and we can't ever miss the
Festival of the Lion King show. My daughter was not even 2 the first time she saw this and absolutely fell in love! It really is a great, great show for people of all ages.
Enjoy your vacation!