My DS is 24, so it's been a while, but here's my $.02.
- You know them better than anyone. Does a thunderstorm on a rainy night send them under the covers? Can a harmless "Boo!" send them to tears?
- Some preparation will be quite helpful - it's often the unknown that children fear the most. For example, stressing that nothing / no one will touch them can make them a lot less afraid.
- I'm not a big fan of coaxing and convincing; IMHO, that serves the parent, not the child. If they kid doesn't want to go, the HM will still be there when they're older.
- This a request from those in the Doom Buggy next to you - or in the same Pirate Ship: if you think that you're going to have to reassure your little one for the entire ride, wait until they're older. I realize that Disney is for all ages, but I've had HM and PotC rides ruined by parents reassuring a young child throughout the entire ride. Hearing "Oh, there's another funny pirate! Hi, Mr Pirate"; "Oh, there's a silly old ghost - go away, you bad ghost" every 10 seconds is NOT fun for the rest of us.