Check in the park when you get there to see what time the fireworks start that night. No matter what your ADR time says, the Dessert Party opens for all attendees 1 hour before the fireworks - and some recent posters have reported that it's actually opening 5-10 minutes before the 1-hour mark, because of the new show projected onto the Castle.
There is no reason to line up 2 hours early; your place in line does not affect your seating. If the fireworks start at 10, you want to get there about a quarter to 9 with your entire party. Check in at the podium and they will give you all wristbands (because if you have to go to the bathroom you have to leave the party area; you can only get back in with the wristband). Shortly before 9 they seat all the checked-in guests; it goes really quickly because they are taking you to a specific table that already has your name on it. The buffet lines open immediately.
As some prior posters said, you really can't see much of the Parade; just the tops of the floats and the pretty lights. If you try to check in, go see the parade, and come back, you'll have missed most of the Party - seems like an awful waste of money. If this is your only MK day and the Parade is a must-do for your family, don't do the Dessert Party.
And for pity's sake, don't eat dinner. The Dessert Party IS dinner. If you have had a full dinner just a few hours earlier, you are not going to want to eat much, and sampling all the wonderful goodies is a major point of the Dessert Party.