Help with Disney World Planning for Fireworks Show!

The Magic Kingdom fireworks are usually at 9PM. You can see them from almost anywhere in the park, but the best place is in front of the castle (and yes you have to sit there at least an hour ahead of time). The dessert on the terrace I am sure is nice. You have a great view of the castle from there and there is a pond separating you from the front of the castle, but you will need reservations for that (call them to be sure).

You should also check out the fireworks and light show at Epcot, it is every night at 9PM in the world showcase and worth seeing.

We just went the day after Christmas. The fireworks at Majic Kingdom were at 10pm. Yes it is crazy getting out of there after them. Depending on how old your children are I would definitely take a rest then go back after dinner for the fireworks. If you can get place to sit for dessert I would go for that option. Have fun and just pace yourself! I had my mom and a 5 year old so that helped a lot, but it is very busy.
A.

You have to make reservations for the Terrace. The times are 7 and 9 usually, with fireworks starting at 9.

Yes, like everyone has said, the fireworks start around 9pm which is probably too late for you to be eating dinner. When we were there several years ago, we had an issue with our park tickets that was the fault of the park. The manager we dealt with while trying to appease us, told us he could put us anywhere in the park we wanted to be for the fireworks, including actually IN the castle (though he did say that's definitely NOT the best place to see them), but that the very best place for us to see them was over in Tomorrow Land in between Space Mountain & the Lilo & Stitch ride. We got to the area around 8:40 & just picked out a couple of benches & he was right, it was an absolutely perfect place to sit, just far enough away from where they were being shot off that we didn't have a pain in our neck from looking straight up, but there are no buildings to obstruct your view between those 2 rides.

which show are you going to?

as in what is the name of the show?
Just below the place you are talking about there's an area down by the water that's a great spot , really there's plenty of good spots Just make sure you can see the castle really well , past main street.

ETA I believe they usually start at 7, 8, or 9. sometimes there's more than one a night. Have you checked www.allears.net ?