Speaking from experience, you CAN miss both Illuminations and Fantasmic and STILL have a magical trip to Disney World! My DS age 3/4 is an early to bed, early to rise kinda guy. He didn't enjoy either Illuminations or Fantasmic, partly because he was nervous and partly because he was exhausted. In contrast, my DD age 10 now could rock the night away! At age 5/6 (before little bro came into the picture), we did all the late EMH and the late parades/fireworks shows. She loves them! Alas, I'll have to find a way to compromise on our next trip. Wishes is our favorite of the 3 fireworks shows. Fantasmic is pretty intense and Illuminations is pretty loud. We also love watching the Spectromagic Parade and the water boat parade.
So, I agree with those who say, it totally depends on your kid!