I'll admit I know nothing about the dessert seating stuff, but we had no trouble at all going to the fireworks and second Fantasmic in July.
Go to the hub, find a good spot for the fireworks in the area immediately next to the entrance to Frontierland. You want to be just inside the hub area, but as close as possible otherwise to that entrance.
When the fireworks end, head immediately to Frontierland and the dock where the Columbia and Mark Twain launch from.
The time we did this, there were CMs with a rope across the Golden Horseshoe to the Rivers of America, keeping us from proceeding, and they made us wait there until the crowd from the first Fantasmic! cleared through Adventureland, then we were allowed to proceed.
There was a good sized crowd, but 99% of the people dropped off pretty early. We went a bit further and found tons of room in the "sitting down only" area just in front of the main soundboard and lighting controls. Waited a bit for the show to start, and it was perfect.
YMMV, this obviously is based on a two Fantasmic one firework type deal, if the evening in question doesn't have that, you may find you need to do something else... and obviously what I'm tossing out here doesn't speak at all to how you'd go about claiming a reservation... but I think the general structure would still work, drop your family off in a good hub spot near the walkway to Frontierland, head over to Fantasmic and get your reservation all figured out, come back, watch the show, then take everyone back to Fantasmic.