We just took my DS in December. He was 20 months old then. I just wanted to say that he really loved all three of these shows - but Illuminations was hands down his favorite. In fact, I think it was his favorite thing from the entire trip. We had great spots right on the railing and he jumped up and down the whole time and said "wow" like a million times. I've never seen him so excited! More people were watching him than the show itself. It was the highlight of his trip!
For the record, though, Fantasmic didn't scare him at all. In fact, nothing scared him on that trip - shows, rides, characters, etc.