Just remember that Benadryl & Dramamine can have the opposite effects in children, ie. make them more hyper than drowsy.
Antihistamine had the opposite effect on DS. His dentist got ridiculous on me and insisted on one for a little bit of work. DS had never been in the main room of the office before, and of course was curious. Dentist decided that the work would be impossible. Forgetting that he does "work" in the private room (where DS had only been until that day). It was AWFUL. DS was fine during the appointment, it hadn't hit yet, and he was in the quiet, private room that he was used to. But once we started for home and for the rest of the day, AWFUL.
I don't recommend giving things like that for frivolous purposes. And flying *behaviour* is a frivolous reason.
Was she in a carseat? If so, is that how she could reach the seat to kick it? If so, gate check the seat. Was she on the seat? Perhaps have her cross her legs so she isn't kicking. Different people have different levels of "appreciation" for being kicked. I had the guy behind me digging into his seat pocket on our last flight, and you can imagine what THAT felt like. If it had continued I'm not sure what would have been my breaking point. I'm really NOT into being kicked, and I have moved mountains to prevent my son from kicking the back of someone's seat, because I know very well how incredibly annoying it can be.
Bring things for her to do. Bring familiar food for her to eat. (try to not bring things containing peanuts, out of courtesy of people who might possibly be allergic, or eat those at the gate where people can get away, rather than on the plane) Don't choose a flight to try something brand new.
I love Ear Planes, if she was hurting from the pressure. DS at 8 has decided he's over those, but still gets a little bit of pain, so I bring Trader Joes Organic lollipops; they don't have anything to trigger him (anything based on corn syrup (note: that's why the antihistamine syrup messed with him, because that's the syrup) and possibly/probably artificial colors) because they are colored with things like beets. The lollipop helps him continue to swallow which helps him not have pain.
The nice thing about the flight home is that she's not new at this anymore. She knows what it's like. And I don't think there's anything wrong with talking her through it...I think that talking about experiences is very helpful, even when behaviour isn't perfect in response to it. You just need to increase your toolbox of things to keep her occupied. Coloring books/pages (does the Merida meet and greet at WDW have coloring pages like the DLR one does?), some crayons... Can you buy a little set of Duplos for the way home?
I'm not sure an aisle seat would help, but maybe a window would? She can look out that way. With an aisle seat she *might* get knocked into by people and even the beverage cart. DH sure gets hit by the cart, and it's several hundred pounds on wheels. Ouch.