Technically, as with all rides and attractions, food and drink is not allowed. If you have a drink, Mickey Bar or other such item in your hand when you approach the entrance ramp, the CM there will ask you to either throw it away or step out of line, finish your snack and then come back.
Now, if you just happened to have a cookie or candy bar in your backpack, you'll probably get away with eating it once you're on the ride. Penalty for being caught though, is being forced to endure the "chili dog sequence" in the Stich theater for 30 minutes straight.
Steve