If everyone is talking about the 1997 movie Titanic that is causing all of the issues with their children wanting to sail, I guess I am a bit taken back that kids that young are watching this PG-13 movie. But that is every parent's decision to make for their family, but had you known it would impact your ability to cruise, would you have kept them from watching it?
We are taking one of the East Coast / Canada sailings - and will NOT be visiting anything to do with the Titanic!! DW Knows what, why and how it happened but doesn't want to be reminded about it and then go back onto the ship. We also don't visit any of the Titanic museums.
Its one of "those fears" that sometimes is hard to talk yourself out of. On the other hand we have visited the Holocaust Museum in DC and have toured Germany ....
My advise to the OP
1) Book the Trip as soon as you are able too (the sooner the cheaper)
2) discuss the issue with DD once - soon, then drop the subject (like others have said)
3) don't tell DD that you have booked the cruise
4) go and have a great time
If you sense continued apprehension around PIF you can choose to cancel
If you decide to sail anyway, you can always "lie by omission" - depending upon how observant your child is she might not even realize that she is on a ship!! When checking in you can say that you are going to a special version of WDW (because
DCL is part of Disney Parks Division, WDW specifically). OK so the muster drill will give it away ... and sorry I am beginning to chuckle to myself remembering what the A-Team did to Mr. T (B.A. Baracus) whenever they needed to fly.