Also at 3 1/2 she's clearly old enough to be "prepared" ahead of time with regard to the behavior expectations and how they might be different from other situations she's more accustomed to. Also try to consider the things that might be confusing and frustrating to her and prepare her for those situations. For example, we have to turn the DVD player off when the pilot says we have to. There is no choice and no option. Don't pitch a fit when we turn it off. We can watch the end of the movie when we get to the hotel... things like that.

She might be old enough to get the speech about "this is very special that we're letting you go on an airplane. We probably should wait until you're older, but we're going to give you a chance now so we can go and do this fun thing with our family (although if its in MKE, that "fun" promise might be an exaggeration ). But, if you show us that you can't behave, then the next time we go on a trip it will just be Mommy & Me, you'll have to stay with Aunt Mary and won't get to have fun."