Last September we got some tasty carmel corn from a cart inside once we already had our seats for Fantasmic. I believe they sold soft drinks, popcorn, & many light-up toys, too.
One of our favorite things to do is munch on hot dogs and popcorn while we wait for Fantasmic to start! There are concession stands both inside and outside of the theater - we usually grab something inside.
No problems bringing food in. My DH has left his seat and gone outside to get something to bring it back. Gives you some options and helps kill the time waiting for the show!
This is one of the few places where I grab a beer before Fantasmic starts. The hotdogs, cotton candy, and popcorn tend to be our favs. Don't be afraid to grab something befoe you get inside the theatre.