Our twin daughters were 20 mo old in December when we went. Though they are twins, their personalities are quite different. Willow is quite fearless, while Tori was scared to death of the Abominable Snowman in Rudolph the Red Nosed Reindeer. Needless to say, we were concerned about how Tori would handle Fantasmic.
To get through Rudolph, we had taught Tori to cover her eyes when ol' Abominable showed up. It was interesting to see her react by covering her eyes during the scarier parts of Fantasmic. It was even more interesting when we realized that she must have been peeking, 'cause the eyes came uncovered whenever Sorcerer Mickey showed up, and the little hands even clapped and pointed at times. Then right back over the eyes when the music turned scary. However, each time the eyes were a little less covered, and I made it a point to excitedly pronounce Mickey's arrival.
Fearless (but very tired) little Willow just wanted to bury her head in Mommy's shoulder and didn't want to look at any of it.
So both survived, and both reacted quite differently than what we expected. Tori's reactions were actually delightful, and Mickey was obviously her hero for many days to come. I gotta tell you, I never noticed Mickey on top of that pole at MGM Studios entrance, but she was thrilled to point him out as we left (of course, it took me a couple of minute to figure out what she was so excited about).
So I guess the whole point is that you won't know til you do it, and some preparation is essential. Give them a tool to help them cope and hug them tight if they need it. Don't force them to sit through it if they don't want to. Same goes for fireworks.. we played a lot with the Wishes and Illuminations web pages, showed them the planning video often, and I firmly believe it helped them a great deal.
By the way, Tori STILL hates the Abominable, and won't watch The WIzard of Oz. Willow loves both. And we get to try Fantasmic again in May...
was 2.5 when he saw Fantasmic for the first time and he loved it 
