I called again on Sunday, 3/14 and was told that 4:30 was the latest available. I asked the CM to check on 5/30 since it is a holiday week end and would have two shows. There are two shows, but still, 4:30 was the latest FDP in the computer. Unless you are going through Magical Gatherings or some other system, you may not actually be getting the FDP. I am only suggesting that you beware and confirm before it is too late.
Alex, Fantasmic is a great show, but be advised that it is a multi-media presentation with pyrotechnics that include fireworks and well, fire. There is a scene with a 25 foot mechanical dragon that sets the lake on fire (it is in front of the stage and in close proximity to the audience) and there are several scenes with loud thunderous explosions. If your kids aren't too young or aren't too frightened by loud noises they will really enjoy it.
There are two consession stands in the amphitheatre that sell hot dogs, pop corn, pretzels, beer, and soft drinks and there are venders that roam the stands with pop-corn, beer, and soft drinks. There are also rest rooms at the rear of the amphitheatre so you may not need to leave once you enter, but you can. You may also bring food into the amphitheatre. You will pass the Anahiem Produce on your way to the front entrance (only non-FDP entrance) to the amphitheatre.