If you do the preshow, something that worked for us with DS2 was to whisper/narrate in his ear right before the lights went down, and then hold him close to us when the lights went off (depending on your child, covering his/her ears would also help prevent the scream scare).
On the whole, taking plenty of time to show your kids videos/photos of the rides before you go on the trip will break down the “unknown” fear factors and help them greatly.