The very back row is mostly open spaces for wheelchairs and
ECVs. The row right in front of that is reserved for the people traveling with those using wheelchairs.
You should not have a problem getting seats toward the back though (and on an aisle). The exit is the same way you came in. In case you need to get away from the noise a bit, there is a set of restrooms at the back of the amphitheater (look for them on your left as you are coming in).
We took my niece and her 3 yr old DD last Spring. Her mom was not sure how she'd react because she was kind of scared of fireworks and loud noises. One thing that helped the little girl a lot was that before the show, we told her what the "story" of the show was in words a 3 year old would get:
Mickey is sleeping and first he has a good dream. Then some of the "bad guys" come into his dream and try to make him have a bad dream. At first, Mickey is scared, but then he says "You bad guys go away. I want a good dream." When he says that, they go away and after that, the dream is very nice.
We told her that sometimes there would be fireworks and even fire, but it was just Mickey's dream, so nothing bad could happen.
She did get scared at times, covered her eyes, covered her ears and buried her head in her mom's shoulder. But, she enjoyed it a lot.
Some of the funny things she said were:
"Mickey really is having a scary dream. I hope he wakes up soon."
"Mickey, tell the bad guys to go away. I want to see the good dream."
"That's a scary monster, but it's not scary to me. Only to Mickey."