My older kids were not really an issue, they loved it, but dd was just shy of three when we last went. I told her about it before we went (she was familiar with the characters and the "bad guys"), so she wasn't startled.
We took her favorite blankie so she had something to put over her ears (she wore it like a hood) and she was fine. The loud noises were lessened by the blanket, and she loved the show.
But if your kids don't like fireworks, loud noises, etc., I might wait. The good news is you can sit way in back and hold them against your shoulder if they are nervous (I saw a few kids fall asleep that way)!