Skip Fantasmic, unless you are sure you little one doesn't mind explosive noises very close by. It is not so wonderful that having your child cower or whimpering in your lap is worth it.
I would recommend HEA, and booking the Tomorrowland Terrace party. The food is so-so but sitting in a controlled area, with a covering overhead is pretty awesome. The view is lovely and HEA starts "gently" with the castle show. It is amazing, we can see it well enough from the Terrace, we felt the noise was even dampened a bit.
Other features we liked best - once checked in, and your table is allocated, you can leave and come back just before the show! You are close to a bathroom, always good with little ones and grandparents. You can walk around and stretch your legs, have a treat and come back to your own spot. Definitely the most civilized way to watch the beautiful show.
FEA Dessert party is better in terms of food and drink but being on the edge of the lagoon may be a bit intense for a sensitive child. Also, in the event of any precipitation, that party is moved indoors. With HEA, you are assured the party is right where you want it to be.