(I didn't read all of the previous posts to pardon me if I repeat anything that's already been said).
We took our kids (4 and 6) for the first time this past October. To prepare them beforehand, we read them some books about flying on airplanes (The Noisy Airplane and My First Trip on an Airplane were two of their favorites) so they'd have some idea of what to expect. We made sure they knew what the security process would look like (e.g., having to place their backpack on the conveyor and walk through the "doorway").
We flew out early in the morning -- we literally rolled them out of bed in their traveling clothes and popped them into the car with blankets and pillows for the one-hour drive to the airport. Getting them up earlier than the usual time was not ideal, and if I could have, I'd have kept them on their usual schedule. I know that some people prefer to travel during kids' sleeping hours "so they'll sleep on the plane," but in our experience, they were far too excited to even consider sleeping, in the car or on the plane!)
Each of them had a backpack for which they were responsible (I'd recommend this for your older child, not for the 18-month-olds, obviously - you'll want to carry the bag with their things), which contained a hat, sunglasses, shorts to change into en route to Orlando, a Crayola Color Wonder Book in the hard plastic case, with the Color Wonder markers (which only "mark" on the book's special paper), a new sticker/activity book (both Disney-themed), and a care package with gum (to help pop their ears), Twizzlers, raisins, mozzarella sticks and sliced apples. Because we were flying JetBlue, which has the TVs in the seat backs with some kid-friendly channels, they also each had a set of headphones.
These items kept them more than busy during our nearly three-hour flight. We thought they might get freaked out by turbulence, but they were nonplussed by it, and really enjoyed flying -- especially when we let them take turns pulling a wheeled carry-on through the airport! I think the whole experience made them feel very grown-up.
Another suggestion: before leaving, we attached a Safety Tat (a sticker-like tattoo that stays on for a few days, upon which you can write contact info.,
http://www.safetytat.com/) to each child's arm, with our cell phone numbers written on it. We instructed them that if they got lost in the airport (or anywhere else on our vacation), they should refuse to go anywhere with anyone, but should ask a police officer, security guard or cast member to call the number on their safety tat. Fortunately, it was never necessary for the kids to use them, but I felt a little bit better knowing that if we got separated, they'd have a quick way to reach us.
