It's important to prepare a kid for an upcoming trip but it's more important to take the kids into consideration during the trip. I mean make sure that you stay at places with pool, or playroom. If you book a train journey there are family compartments. Allow extra time to see sights.
I have 3 kids under 6 and Mr. Nacho and I used to see a city and the outskirt plus shopping in 48 hours at Pre-kids time, now to do the same thing it takes 5 days - playground stop, eating stop etc. things take time with kids.