I've done the Fantasmic! dessert buffet, and really enjoyed it - definitely the best view of F! I've ever had, since we were up high and could see everything! For us it was worth it, but I don't know that I'd want to fork over the $$ to take a three-year-old...As RMH noted, it's really nice NOT to have to stand/sit around the Rivers of America for a couple of hours in order to have a decent spot to see the show.
Also, be aware that you have to book this 30 days in advance, and there's only 21 or 22 spots on the balcony, and they sell out FAST. People call right at 8:00 when they open, and still get shutout sometimes. There's also a seating section down by river, where instead of a dessert buffet you receive a box of desserts. That accommodates more people, but I think the price is the same. That area also can be reserved 30 days in advance, and regularly sells out.