This is a question only you can answer for your child. Why?
My son has grown up in amusement parks and WDW (first trip at 6 months old, 8th trip in a few weeks) and he loves Fantasmic
(he is now 3) and always did. My niece's child at 5 years old was terrified, he closed his eyes and held his mother close as he could all show.
Only you know your child's fears and fear limits. As a general rule, I have seen many young toddlers enjoy the show, I have also seen quite a few leave early.
I know that this doesn"t really help, but it is an honest of an answer as I can give.
Each child reacts differently to each ride. In fact, I have seen my son go from loving an attraction to being afraid of it back to asking to go on it again and again. Children are very fickle.
I can only suggest going to see the show and sit near the top, it will be less threatening up high and if need be you can make a quicker exit.
Good luck. Remember to be patient and allow the child time to grow into WDW and the magic. Do not write off any ride as too scary or assume any ride to be good.
Try to go into your trip with an open mind and allow your child to make suggestions of what to see and do. Many people forget that their child is a big part of the trip and the main reason they chose Disney as their vacation destination. You will be surprised how much a child does understand and know what they like to do. Get maps for all parks and look over them with your child. My son picks out what he wants to do at each park. sometimes what we find BORING (Dinoland dig up the bones!!!) they will love and remember for a long time.
Hope you and your family enjoys the trip.
