If there is a second show, go to that one. Many families with young children go to the first showing, then head out as the park usually closes when the second show starts.
If you want really good seats, in the center, towards the front, then GET THERE EARLY. I cannot stress this enough. As it will be a slower time of year when you go, it might not be as critical, but you don't want to get there too late. Last time we went, we hung around shopping until we saw the line begin to move, then hopped in with the crowd and walked right up to the ampitheater.
However, seats are good ANYWHERE in the theater. We usually sit on the far right side, and the show is just as beautiful and breathtaking as it would be in the middle. So if you don't get center front seats, don't strees; your view will still be awesome.
The show itself is 30 minutes long. A warning: people around you may start getting up to leave before the show is over. Don't follow them; you want to see the end of the show, because its the best part. They are trying to beat the crowd, but if you just sit for a few minutes after the show is over, you can get in towards the back of the pack and take a nice stroll out into the park. The park will be closed by now, but it will be nicely lit and relatively empty. The shops are usually still open too, so my family shops our way out, then goes and hops on a tram/bus.
The trasportation will be crowded, but if you wait a little while the initial crush will be over and you can get in a line thats long, but not too crazy. Again, we usually go at peak seasons, so it may be a little less crowded for you.
I hope this helps! Fantasmic is a great show; one of my favorites. It will be great for you to see.:wave: