I swear both the computer system and the ADR CMs have more trouble with this event than any other one on the property.
No matter what anything says, there is only one Dessert Party per night and it starts 1 hour before the fireworks. It is worthwhile to get there 5-10 minutes early to pre-check in, but getting there any earlier does not get you a better table. Tables are assigned when the reservations are made. At the 1-hour mark the event opens. CMs immediately lead you to a table that already has your name on a placard on the table, and you can start to eat.
There is no entertainment; this Party is all about the food and the fireworks. To me it is more for adults and older kids rather than the under-8s, depending of course on your child's palate. It has lots of little, fancy, mini desserts to sample. Think chocolate-covered strawberries, not Mickey bars. The beverages are also self-serve and include hot coffee, iced tea, lemonade, and milk. Soda pop is not an option and for some families that can be a deal-breaker (for us, bottomless glasses of ice-cold milk are one of the party's biggest plusses!).
The food is available until at least 10-15 minutes after the fireworks are over, although certainly by the time the fireworks start the CMS are reducing the buffet down from multiple lines to just one table. They will also start clearing the individual tables once the fireworkst start, but are happy to leave your plates and glasses if you tell them to.
You do not have a clear view of the parade before the fireworks, and if you try to go to Main Street for the parade and then run to the party you will have missed like 75% of it by then; if the parade that night is a deal-breaker the Dessert Party may not be the best choice for you. It is not cheap and they do charge your credit card when you make the ADR. It would certainly be less expensive to buy yourself a great big ice cream cone and a soda out on Main Street. But to me, part of the attraction is the chance to try all the different kinds of desserts. The environment itself is much more sedate than Main Street. You are sitting at your own table on real chairs with enough privacy to talk to the members of your group without shouting. It's a nice chance to relax and process the day.
When the fireworks start many people just automatically gravitate to the front railing, but you really can see just fine by sitting down too, and there is enough space for everybody. Have your camera ready: Tink flies from the castle to RIGHT ABOVE YOUR HEAD! And after the fireworks end, go refill your drink and sample one last cookie. Waiting just 5 more minutes lets the worst of the crush move out on Main Street. If you decide to go, have a great time!