The premium viewing section, which is between Haunted Mansion and Tom Sawyer's Rafts is first come first served. About an hour before Fantasmic starts, the CMs will take your tickets and write your name down and the number in your party. About a half hour before Fantasmic they'll start seating you in the order that they have the names down. By that time, everyone in your party will need to be there or they won't seat you. We did this last August, and that is what happened to us.
Balcony seating is the best, but premium seating is also good, especially if you are going to watch the first show and do not want to wait for 2-3 hours to get a good spot.
I believe pricing for premium seating is $53 for ages 10+, and $43 for 3-9 (2 & under are free if they sit in your lap). Balcony seating is $53 for all paying guests (that is, 2 & under are free if they sit in your lap).