Watching the fireworks will cap off your birthday celebration beautifully! My two favorite restaurants from which to view the fireworks at the Magic Kingdom are O'hana's and California Grill.
O'hana's is located in the Polynesian Resort and the California Grill sits atop the Contemporary Resort. Both are conveniently located on the monorail from MK. Although I knew it wouldn't be guaranteed, I requested a window table when we checked in at the host desk at O'hana's. During our meal, the lights grew dim, choreographed music piped in over the speakers, and created a wonderful memory for our family.
The view of Cinderella Castle is magical inside the California Grill. However, the private outside balcony for restaurant patrons could be a romantic setting as well as a family-friendly choice for your celebration viewing. It's an option on par with inside-viewing, especially if your reservation does not align with the fireworks, as would the pier at the Polynesian.
I recommend checking the
park calendar at disneyworld.com to help you estimate the best time to select a reservation.