For our kids, we had a box "delivered" on our side porch that, when opened, had two helium Mickey ear balloons that floated up (tethered to the bottom of the box so they wouldn't get lost on accident), a postcard/invitation from Mickey and the gang, and "Guide To the Magic For Kids," a picture book all about the WDW attractions that are of greatest interest to kids. (The most recent edition was 2016, if I look on Amazon, so anyone purchasing it would need to be prepared to update it to reflect discontinued attractions. There are also a number of other kid-geared WDW books out there that are probably more current.)
We made our "announcement" a couple of months ahead of time, for several reasons. First, our kids love to be a part of planning family trips, and we wanted them to have a chance to learn about what WDW had to offer before we went. (How could we expect them to get all excited about going to Disney World when they have no idea what's at Disney World to get excited about!??!) We wanted time to "educate" them with bedtime stories and films about what they'd see. (For example, at ages 4 and 6, they knew nothing about the Swiss Family Robinson, Tom Sawyer, Dumbo, Pinocchio, The Three Caballeros, etc.) We also gave them a little "training" about what to expect when flying for the first time -- we read Richard Scarry's Busy Busy Airport, and The Noisy Airplane, and talked about what to do when going through security. (It worked amazingly well -- our kids breezed through the TSA lines with all of the efficiency and speed of seasoned business travelers!)
The day we left, we surprised them each with a "care package" to take on the plane with them. They had headphones (we were flying JetBlue, with seatback screens), Disney activity books and washable markers, Chapstick, sunglasses, gummy treats and granola bars. If we had it to do over again, I'd have included autograph books and markers for use on the trip, as well: that was one thing we didn't think we'd bother with, but once the kids saw some of the meet-and-greets, they were all about it. We didn't want to spend tons of time in line for characters, so we let each child choose 3 characters that were "must-meets," and made sure those were on the schedule, with other meet-and-greets added in only if the lines were short.